Can anyone help me?

I am getting back into bowhunting after a few years away. I bought a new Bear Vapor 300 about 8 weeks ago from the local pro shop. They set it up w/ Beman ICS arrow and an NAP 2000 fall away rest. I started shooting at about 62# at a distance of 25 yards. After awhile (200 shots or so) I decided to increase my poundage to 65#. All of the sudden my groups dropped about 6-8 inches. I took it back to the pro shop and they said the string must have stretched. Stating it wasn't uncommon for the string to stretch at about 200-250 shots. So they adjusted my nocks and sent me on my way. After I got it sighted in again and kept practicing, I was shooting consistent groups of 2 inches or less.........except on problem.

My groups started slowly moving lower and lower on the target every two to three days, and I have to keep adjusting my sights each week or so. The pro shop checked my bow over, said the rest was fine, and then said "I guess the string is still stretching".

Is this normal? For the string to consistently continue to stretch? I don't remember having a problem with this when I bow hunted previously.

Is this to be expected? Or am I being given questionable advice because the pro shop just doesn't know what is wrong with my bow? I would appreciate everyone's help.