I live in PA as well (Central) and have had great sucess with the can here. I don't necessarily think you are doing anything wrong, and Jim's advice is excellent advice. Grunting in conjunction with bleating with the can is a very effective way to call bucks here in PA, especially the last 3 weeks of our archery season.
The volume of the can is not real loud so keep in mind that a deer is only going to hear it if it is within hearing distance. How far the sound carries depends on a lot of variables including wind, distance of the deer, if the deer is walking when you call, etc. Point being that a deer has to hear the call before it can respond so your stand location can be a very important factor in determining if your calling works. Said another way, a deer needs to be fairly close to hear the call.
Regarding actual use I generally vary how often and how many times I bleat with the can. Just like with turkey calling you should mix it up a little rather than do the same thing over and over. Sometimes I'll bleat 5 or 6 times in 30 seconds. Other times I bleat only once or twice. Also, the time between calling can vary as well. You might call now, and then call again 5 minutes later, and then wait 20 minutes before calling the next time. The point is don't follow a strict schedule (i.e. every 20 minutes)...that is not natural.
As Jim mentioned, grunting along with the can is a great way to call buck. I usually do a little bleating followed by 3-6 grunts. Vary your grunts (long vs short) and try to sound like a buck following the hot doe that you're pretending to be with the can.
I hope this helps. Good luck.
-Eric