If they're brand new they will need several washes before use so they get better absorbency - you need to dry them between each wash. With second hand ones I would be a doing a strip wash of them.

Wash them using a mild wash mix - liquid if possible - using about a third of what you would normally use. When you start using them you will need to do a rinse cycle first and then a full wash. I used to do an additional rinse at the end as well, but I think that's more to do with my useless machine.