Experts and BHG readers answer.
It's really hard to guess what's going on here. It sounds like you are taking good care of it. I do wonder if anything else might have changed. For example, does this plant receive increased root competition from nearby plants or a now-mature tree overhead. That can often be the cause of problems you don't think about because they happen so slowly. It's also possible you are overfeeding the plant or that it's getting blasted by winter winds. You don't say how old this plant is, but if this is a very old specimen it could be slowing up due to advanced age. I just can't guess on this one without seeing the plant and situation, but I hope I've come up with some possible solutions.