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.
(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.