We have tiled on floorboard overlaid with chipboard which was screwed down, then sealed with pva. This is a kitchen floor, so we have used quarry tiles.
We used a flexible tile adhesive + grout to fix the floor tiles, but we couldn't use this to grout because the adhesive marked the porous tile. So we used a cement based waterproof floor grout, to grout between the tiles. In some places this just keeps crumbling. we tried mixing pva with the grout to see if this would help, because the tile and floor are no longer porous, but it has'nt.
Is there any sort of adhesive of another sort of grout which i could now use. I would appreciate your advice, and so would my sister, whose floor it is. Thanks Mary North.

Hi Mary, welcome to the forums. I am sad to see you are having problems.

Unfortunately you have several things that is causing your problem.
1. It is normal to overlay with Plywood not chipboard.
2. PVA has no place in tiling. You should have used an acrylic based primer that soaks into the substrate and dries. Your pVA will have left a very thin laye ON TOP of you surface to which you have tried to fix your tiles. PVA becomes active again when it gets wet so is not a stable surface for tiling.

What you should have done was - over board you floorboards with 18mm WBP ply, having sealed the back and sides of the ply in accordance with your adhesive manufacturers instructions, primed the top surface if necessary, again this depends on the adhsive you used. You then should have used a good quality highly flexible adhesive, NOT a combined adhesive/grout as you have found out, and then grouted with a flexible grout.

I fear the only real solution to your problem will be to start again. This will be quickest and give better results.

Sorry to be the bringer of bad tidings.

mix the grout with some sugar and pop some apples under it,,slam it in the oven and bobs yer uncle

Good post from Grumpy there Mary. I fear you have been shopping at a DIY chain and not a tile shop, they may have been able to advise you of this in the first place.
The adhesive & grout premix is usually a disaster waiting to happen, unfortunately in this case it has happened.
Who is the manufacturer of the mix and who advised you to get it, might be worth presenting them with the problem.

it's a shame you didn't find this site earlier Mary, it looks like a strip back and start again, I just hope it wasn't a large area.