There are loads of things that it
could be.
The first thing that I'd check is that the spring on the front of the hop unit is still there, and pushing the hop back against the geearbox.
Then I'd check that the nozzle is secured in the tappet plate, as it's easy to miss this when re-assembling. Ensure that the nozzle isn't either loose, or stuck, i.e. that it can be pushed back under spring tension, and then returns by itself, but that it can't be wiggled back and forth easily.
Check that there's a good bit of puff coming out of the nozzle.
Either way, unless it's the hop spring, you're going to have to go back inside the gearbox to see what's gone wrong there.
Be sure to check that you've got good air seal on the piston, cylinder, cylinder head and nozzle
before reassembling, but I'm sure you'll have learned that lesson.