I don't know whether or not this is intended to be this way, but i'll report it anyways just to be sure.
When you place an impulse command block and put a command in it and then activate it, it works. But if you keep it powered and you change it from impulse to repeat, it wont trigger. It will trigger once it is turned off and turned on again.
Additional description from @unknown:
To clarify the description a bit, this occurs both if the block is set to "Needs Redstone" and is receiving power, and also if it's set to "Always Active". It's also worth noting that this is not fixed by updating the block, the setting must actually be changed back to Needs Redstone, then back to Always Active.
This bug also occurs with impulse mode. Set it to Chain and Always Active and type in a command. Then change it to impulse. The command will not be run until it is changed to and from Needs Redstone.
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also occures with chain -> repeat, not only impulse -> repeat
Effects all 1.9 pres, 1.9 snapshots, 1.9.1 pre 1, and 1.9.1 pre 2.
The enviroment is supposed to contain pc details.
Confirmed for 1.9.1 pre release 3
To clarify the description a bit, this occurs both if the block is set to "Needs Redstone" and is receiving power, and also if it's set to "Always Active". It's also worth noting that this is not fixed by updating the block, the setting must actually be changed back to Needs Redstone, then back to Always Active.
This bug also occurs with impulse mode. Set it to Chain and Always Active and type in a command. Then change it to impulse. The command will not be run until it is changed to and from Needs Redstone.
This bug is also in 1.10.2
Video: https://www.mediafire.com/?62mb0zbpb6r0o6b

I can confirm this is happening in 1.11.
I would set a command block down, type in a command like: say @p Hello :and put it to repeat and always active. Then I would let it run for 2 seconds and then I would go in and change it to impulse and I expected it to run the command immediately after that, at least once, but got nothing. Went back in and changed it to needs redstone then powered it. And it works. Also I went away like a few blocks and went back to try another command block i had sitting around (it was working perfectly before) and it stopped working. Haven't a clue why. The command for that one was: testfor @p[x=1050,y=56,z=-449,r=1,score_Adult=1] :And like I said it was working perfectly before, and changing the command block nor changing it's settings works. It seems the testfor command is a little finicky too but idk.
confirmed for 18w03b.
Still affects 19w04b.
I have the same problem when using a chain command block and switching it to "Repeat" via the UI.
It works after setting to "Needs Redstone" and back to "Always Active"
[media]
Version: Minecraft 1.14 Pre-Release 5

Still in 1.14.1 Release

Still in 1.14.2 Pre-Release 1 and 1.14.2 Pre-Release 2
Still in 1.14.3
Here's an extract of MC-156789, it's a duplicate post, but it has a lot of information about the problem:
Here is a list of commands in repeating command blocks that does not work:
(Boosts) If you are in a certain team and crouch and that your boost score is 0, you get an effect:
Command#.1 (Repeat): /effect give @a[team=Speedster,scores={Crouch=1..,Boost=0}] minecraft:speed 8 13 true
Command#.2 (Chained): /scoreboard players set @a[team=Speedster,scores={Crouch=1..,Boost=0}] Crouch 0
Commands that says when the boosts are on or off (the "Boost" scoreboard is affected by eating rotten flesh): (these 4 commands run in a loop)
Command #.1 (Repeat) /tellraw @a[scores={Boost=3}] {"text":"Boosts ON!","color":"dark_green"}
Command#.2 (Chained): /scoreboard players set @a[scores={Boost=3}] Boost 0
Command #.1 (Repeat): tellraw @a[scores={Boost=1}] {"text":"Boosts OFF!","color":"dark_red"}
Command #.2 (Chained): scoreboard players set @a[scores={Boost=1}] Boost 2
Here is a list of commands in repeating command blocks that does work:
If you walk on light blue concrete, you get a speed boost:
(Repeat): /execute at @a if block ~ ~-1 ~ minecraft:light_blue_concrete run effect give @p[distance=..3] minecraft:speed 1 15
Mobs have special effects:
(Repeat): /execute at @e[type=creeper] run effect give @e[distance=0..1,type=creeper] minecraft:blindness 10 0 true
From what I could see right now, the only commands on repeat that does not seem to work are the one that needs a specific criteria/another scoreboard objective to be activated (for exemple, having {boost=0} or {crouch=1..}).
Also, the bug is still happening in Snapshot 19w35a
Have you tried placing a block next to it after changing it to repeat? This issue may be related to quasi connectivity and working as intended.
@o62 I tried placing a block next to it after changing it to repeat, but it still didn't work.
This is happening again in 1.17.1
Ok I think I know what's happening:
When changing command block options (well, for now on: impulse to repeat mode), they don't consider their new options until they are redstone updated.
Also, if you do this and the command block is set to "Always active", you need to put it to "needs redstone", press done and set it back again to "Always active" to consider his new state.