According to what I have read about acrylic stamps, there is usually a residue on them left over from the manufacturing process. To removw that residue so that the ink sticks, there are several things you can do to condition the stamps:
1. wash stamps in warm water and a gentle dish detergent
2. rub the surface of the stamps with a white eraser
3. use an emery board (like for your nails) or a VERY fine grade of sandpaper and GENTLY rub the surface of the stamp just until it looks cloudy (not as shiny as when you bought it)
4. use Versafine inks and chalk inks rather than the usual dye inks.
HTH!