You can, but you should leave it for a bit. Try this:
1) Apply foam in dabs on scalp
2) Lightly drum fingers on the scalp where foam has been applied - it will get to the scalp faster and more efficiently this way
3) Wait 2 mins for foam to dry
4) Move hand lightly through the entire scalp - this will give your hair more volume
5) Comb/style hair
If your foam is watery then:
Here are my tips for keeping Rogaine foam foamy:
1) Shake can
2) Place can under running cold water, then keep tap running and:
3) Put can in the sink plug hole so that it acts like a plug
4) Fill sink to a level halfway up the can
5) Place fingers in the cold water for a few seconds
6) Remove can from plug hole and shake can
7) Dry can and fingers on towel quickly
8) Apply thick, foamy Rogaine foam