There might be just some dirt in there. Unscrew the top and flush the line out, and clean the riser with some water while gently pushing the seal cap up/down. If there is any debris in there, this will get it out. While your at it, you can clean the filer too.
If this doesn't work, then the best bet is to just replace the sprinkler as there cheap enough. Sometime dirt will ruin the riser with scrapes and rough areas, and make it stick constantly.
I'm no expert..........YET!

I just like to suggest things and learn...

See what the pro's have to say first.