The problem could be your tap water, on the other hand, it may be the developer.

If you boil the tap water and let it cool, that will get rid of the dissolved oxygen (which may cause problems) and it should also get rid of some of the chlorine.

On the other hand your water may have a low pH (i.e. on the acid side) and that will slow or stop development. You could test your water if you have litmus paper or an aquarium pH meter available to you.

IMHO, the simplest explanation of your problem is that your developer (TD-76 and FX-50) is going bad.