i would say it is normal. some crosses will not take because of the ploidy issues. diploid, pentaploid, etc. I'm not really educated much on the genetics, but rugosas would be an example of a diploid, and a canina rose would be a pentaploid, which is difficult to cross because of the uneven chromosomes. Some roses are said to be sterile, like stanwell perpetual. Some roses have traded fertility for extra petals, some have low fertility period. the hybridizing forum would have better answers on those issues. of course the easiest thing would just be to try, and if it doesn't work, so what. Some hips may only contain a couple of seeds and some may have several.

Thanks for the information, len. I'll try to the other forum like you said.