I wish I could help. I've read (MB's site, maybe?) that wax is not necessary and maybe even structurally undesirable. Could they object to something in the wax?
Out of curiosity, if they have refused to build in this for a matter of months, what have they been doing?
A wax coating on the wood will sometimes prevent the bees from building on the bars/frames. Bees are used to working with bare wood, trying to outthink the bee by pre-applying wax to the frames is self-defeating. The wax will actually make it much harder for the combs to remain attached to the frames as it prevents the "anchoring" that bees like.
A second consdieration is population. If there's not enough bees in a hive to force the population off of one frame and onto another they will not build comb. Bees build comb only underfoot. In a case of population comb building can sometimes be finesed by moving 1 or 2 drawn frames away from the main group and installing an empty frame in place. As the bees draw back the cluster they may be motivated to drawn comb on the empty, but now occupied, frame.