I tried several different kinds of spring compressors when I had to change my valve springs in the car, and found that the kind that is C-shaped and has ratchet mechanism that pushes on the valve head gave me the most room to get the valve keepers in or out. Most of the ones that try to grab both ends of the spring wouldn't line up correctly to work.
You need to put some tension on the compressor, then tap lightly on the side of the valve spring retainer to get the keepers to release. You also need to do something so that if the keeper releases suddenly and drops off, that the keeper doesn't make a beeline for the oil drainback holes in the block casting and wind up in the oil pan, leaving you one keeper short to put it back together again. Been there, done that; I had an extra, so there is a keeper in my oil pan that has been there for 8 years or so!
Marty