If you're using tubs of lime putty, just make sure you pour off the water first so you're just putting the stiff putty into the mix. Provided the sand isn't completely sopping, you should be OK. Dry sand and lime putty make an ideal mortar, but plaster does need to be more plastic, so the extra water in the sand could be doing you a favour.
I dig up my own sand as my garden was in the sea a few thousand years ago. In wet weather it is wet so it's just right and in dry weather it is dry so I add a little water. I have to sieve out the oyster shells as they don't make for a smooth finish on plaster.
