Hello Christelle, you want to repaint on an undercoat to which was pasted a wallpaper and the paint that you tried to apply does not hold. You are probably right, there must be glue! It is best to lightly sand your old undercoat, then reapply a new undercoat before permanently covering your walls with the paint of your choice. This will be long, it is necessary to take the time to redo all the steps for a better result. Do not forget Christelle to always respect the drying times between each layer also, it is essential for a good performance of the paint.