I would be concerned that something is "wrong" -but that is just me. Oddly enough I expected pain with this shoulder replacement - everyone told me it would be very painful - but I did not have any pain or discomfort at all. The exception was that the lidocaine like nerve block I had that was inserted in the nerves in my neck for two days plus some hours post surgery was uncomfortable. Also it made my left hand and fingers numb. That panicked me a bit.
You might ask for a blood test to determine if you have inflammation (C reactive protein, Sedimentation rate).
How long does the ache last? Have you tried rapid release tylenol gels? Celebrex and also Tylenol work for me. I did take Celebrex about a week after my surgery because my "native" right shoulder was starting to give me pain from being "overworked."

Do you sleep in a recliner? I had to sleep in a recliner until the sling came off. It was not a good experience but, again, I did not experience any pain because of it.

I would think that a dull ache would go away but I would pursue it - definitely.
I keep telling patients that they need to have any orthopedic surgery done by a a surgeon that specializes in doing just that body part. A shoulder surgeon should only be doing shoulders and elbows - and maybe wrists, if he or she is trained. Similarly, for a hip or knee shoulder -- that should be their only focus.
I do credit my pain free experiences to do just that - a dedicated Mayo shoulder surgeon and a dedicated Mayo hip and knee surgeon.
Good luck and keep us posted.