dispensers do not function with a fast redstone clock

What type of clock did you use? Could you provide a photo of the setup you used ?

I was able to use a comparator clock just fine.

This has been brought up a few times, but it's likely there needs to be a longer line of redstone between the comparator and the dispenser to allow enough drop in current. (MCPE-19390).

Please add screenshots and more information if this is not the same issue.