Hotpoint Aqualtis wont fill then displays flashing f5 code?

I have a Hotpoint Aqualtis AQ9D 69S. The pump filter was blocked, so I cleared this - no joy. Next I checked pressure switch pipe, seems clear but I thought I'd try putting a small piece of pipe on the switch inlet & blowing, suddenly the fill switch operated. I replaced the switch with genuine part, still no joy, but I discovered that if I pause the cycle, manually open the door and put some water in the drum, the fill operates as normal. As soon as the cycle operates the pump and drains the drum, it's back to square one, fill won't activate.
Help PLEEEASE! It's driving me crazy......:-(