I have had a poppet plug at the start of a transfer. It doesn't take much, but once cleared (by removing and reconnecting) it was fine. It is hard to tell how fast the beer is flowing, so I used to put a piece of tape on the fermenter to mark the level so I could judge the change. Now I put the keg on a scale and watch how fast the weight is increasing.
I've used the scale method too, but more or less to tell when the keg is full. It's never been a perfect science and my batches vary (trub) depending how/how much I hop so I kinda gave up on it. It is nice to use for seeing how the transfer process is going. I didn't know my transfer was stuck until I came back 30 minutes later. The scale would have paid off there.
I did find some suspicious material in the disconnect when cleaning. This was _after_ running oxiclean and purging the transfer hose, so not sure how much gunk was in there originally. Most likely where the problem was.
![](https://imgur.com/a/4ccadYQ)
(I can never get pictures to post in the forum, so here's a URL
https://imgur.com/a/4ccadYQ). I've been experiementing with just tossing hops in the boil but might go back to using the spider with larger hop loads.