Could be your water, is it hard? Or it could be that your washing liquid in fact contains conditioner. It could be worth trying to add some white vinegar to the rinsing cycle or try soaking them in fabric softener or borax solution and then wash perhaps with some gentle washing powder or liquid but do check the pack carefully.
It might be best to tumble dry just briefly to try and fluff the fibres up and then dry naturally.
Good luck