Most mats are made with a `non-slip` coating, though for some people this turns out to be even more slippery than non. If it`s a problem, follow these WASHING INSTRUCTIONS:
Put your mat through the GENTLE CYCLE in your washing machine, perhaps with a little bit of wool wash (softly, etc), and then hang it over a clothes line, clothes horse or even a door to dry - it’ll take a good couple of days to dry - they absorb a lot of water (and are hard to squeeze out).
That should fix the problem.
If you find your hands are still slipping on your mat (downward dog pose especially), make sure you give your hands a good soapy wash before your class to get rid of any body lotion/hand cream residue.