Don't forget that a "symptom" of getting fit is a lower heart-rate when exercising. Like MrS says - is it your increased fitness?

The only other reason where HR doesn't go up that I can think of is when you are getting sick... It will go up as expected but reach a plataeu earlier than you think it should and it doesn't matter what you do, it just will not go higher - mind you, this usually goes hand in hand with an elevated resting HR.

I seriously think it is most likely a sign of your fitness level... you are not sick, you are on mo meds, you have been "pushing" yourself on some of your training rides... I think your body has responded to your requests and increased its ability to cope with stress (ie, exercise) more effectively!