i've had folding tires slip several times. Folders can often have an almost textile surface to the bead, which can be a bit slippery. Increasing the pressure will usually fix it. If you don't like what that does to the ride characteristics, my 1st option is to use some stuff called liquisole(a shoe repair gunk).. Smear a thin layer on the bead and allow to cure. Particularly when new, this surface will be really grippy.
If that isn't enough, get some glue used for moutig tubular tires to rims. Glue one of the tire beads to the rim. IT'll be enough to prevent slippage, while still allowing access to the tube in case of punctures.