In addition to what everybody else has already suggested, I've found that Vietnamese ground cinnamon and some freshly ground nutmeg will go a long long ways in tweeking that flavor you might be looking for. I suggest staying away from any ground cinnamon found in your common grocery store...pay a few extra cents per bottle and get the highly aromatic and powerful stuff found at a specialty store.
Out of curiosity, did you use can or whole pumpkin? I've found that only a rare few (and congrats to those of you who are those rare few) can get a strong pumpkin taste from whole pumpkin.
It's Vietnamese cinnamon--a recent gift from a friend--really nice. I was thinking about grating some fresh nutmeg since what I used was probably a year old. My allspice is very fresh.
I used organic pie pumpkins bought that morning at the farmers' market. I quartered them, microwaved them, scooped out the flesh, then put the flesh in the kettle, not the mash, and I also simmered the skins to make a gallon or so of "pumpkin tea" to add to the sparge water. The pumpkin flavor is elusive but there, but the ale definitely has that nice mouthfeel I associate with pumpkin beer. (I may be the only one who detects this, but I pick it up in every pumpkin ale I've tried.) I'm assuming my SG was a little higher than expected because of the natural sugar and carbs in the pumpkin.
I agree that canned pumpkin can taste more pumpkin-y...some people take mashed pumpkin and cook it down again to get a more condensed flavor. If I were to do that, I'd put it in a baking pan and bake it at 250 degrees for a while, stirring now and then (sorta like making baked apple-butter, for anyone who's done that--my grandmother made apple butter that way). I cooked and froze more pumpkin this past weekend, so who knows, I might do that... perhaps a Thanksgiving morning brew, since I'm not cooking this year. Or I could use canned pumpkin... only my brew friends would know. :-p