Sounds like you need to replace the guards. In fine or for that matter any grass good guards and sickles are very important. There are guards out there that have adjustable hold downs, these keep the section cutting firmly against the guard bottom.
All of my customers using sickle machines in grass put the factory style hold downs solid all of the wayacross the cutter bar. There are two different ones because of the overlap of the sickle in the middle. One side has a higher bend int it to compensate for the center overlap.
have you looked into a disc swather? However, i would be concerned about shatter loss if in a high value grass seed crop. I always hated getting into swamp grass at the bottom of the field, made the machine vibrate badly sometimes. Just some FFT.