We have recently been implanting tumors made more antigenic by expression of OVA. WE are finding the growth slows or regresses in some animals. Particularly, B16.F10.OVA in c57Bl/6 mice
I'm not sure why you would be getting these results. Check your protocol with this one (http://altogenlabs.com/xenograft-models/melanoma-xenograft/b16-xenograft-model/), and that should help you find any clear errors. You'll notice that it does take some time (around a month) for a full study to be run - slow growth is kind of expected in B16 hybrid cell lines.