What causes shares to be rejected?

[DevaVictrix]

I have been mining MUSIC for the last few weeks at 60MH/s. This should give a return of 200 coins a day. But this morning I added up how many coins I got yesterday, then the day before, and a few more random days from the past. Turns out I'm being rewarded only 130-150. Substantially less!

I was using Genoil's ethminer. So I switched to Claymore's and it is telling me that almost every other share is being rejected.

Are there any steps I can take to reduce the number of rejected shares?

Thank you