Floor tiles

We are renovating our bathroom, totally gutted, the floor is a very dense type of grey/green chip board quite old at least 15 years but level and firm over the old floor boards having put new floor tiles on this board using flexible adhesive supplied with the tiles we have found that after 4 days i can lift them very easily the adhesive has stuck to the tile but not the board any suggestions what have i done wrong or is it the board, is it worth taking up this board and fitting tile backer board do they sell it here.