Invalid shares - high percentage. What can be wrong?

When mining I sometimes get invalid (rejected) shares.

Mining on CPU with nheqminer for Coinmine.pl, although I had invalid shares on flypool too.
I'm having 17% of invalid shares according to stats.

But now I looked at ROUND SHARES statistics of Coinmine, and see that many guys are having from 5 to 50% of invalid shares.

Is something wrong with miners, hardware, network or smth? How can I fix it?

I guess that (and also the 20% founders tax) can be the reason of earning less than calculated.

Bad internet upload. Losing packets