exp orbs float around and do not get absorbed

after killing cave spiders from a trap I am unable to collect the xp orbs. they float around or sit at the bottom of the trap. There is a hopper at the bottom to collect all the drops.

What I expected to happen was...:
xp orbs are collected increasing my level

What actually happened was...:
orbs fly around and do not get absorbed

Steps to Reproduce:
I followed this design for my cave spider trap. only added the hopper and some half slabs around the hole because the spiders could get by the fence post. (I will be opening up another issue for that)