| if they are tubless tyres u need to seal the bead (around the edge) of the tyre to the rim
a compressor should do it no probs if not you can punch the top of the tyre a few times it may help them seal, a little soap & water around the inner edge might help it seal & slip on better
if its a tubless valve it may be leaking u will need to replace it
or the bead of the tyre may be damaged this will cause a leak.
another thing tubless tyres dont alwayse work on chinese rims, the casting may be pourous or just rough as guts & will leak,
.. a easy fix is to paint the inside of the rim with rubber or a latex sealant , or a few coats of paint
if they have tubes the valve may have moved if you rode the bike when it was almost flat & tore the valve from the tube | 
