I’ve just had exactly the same problem on Playtime. Underneath the water pump, on the inlet side between the water tank and the pump, is a mesh filter. This appeared to be letting in air (there was a very soft but audible ‘ticking’ noise coming from it) but strangely not leaking water. I dismantled it this morning and found that one of the 4 lugs holding the 2 halves together was broken and the others were not properly tightened. After a quick clean I have reassembled it and fully tightened it and the problem appears to have been fixed (fingers crossed). If I come across a replacement (with 4 lugs intact) I will buy it to have a proper spare. I have also considered buying a spare pump. If the one fitted fails, how do you get any water out of the tanks . . . . . ?

If yours is the same problem it is easily fixed! If not, then good hunting!