Bench Press and Shoulder Press Stabilization?

I'm currently doing Layne Norton's PHAT style training program. I notice that in Layne personal workout layout he does dumbbell bench press AND dumbbell shoulder press on his upper body power day. Today I did barbell bench and when I went to do the dumbbell shoulder press my stabilizer muscles were TAXED. I'd estimate that I had the strength to shoulder press with around 70lb dumbbells, but on my 45lb dumbbell warmup set it was ridiculously hard. I barely felt strain in my shoulders because the weight was so light, but it was SO hard to stabilize the dumbbells even on the long vertical bench. In the past I've always been a dumbbell junkie to develop my stabilizer muscles. Are mine just weak, or is it very typical for this to happen? I was forced to do machine shoulder press which is fine, but I just want to know if I'm missing out on something I should be working at.

Thanks in advance!