It’s totally normal.
Being able to concentrate at Mass is a special consolation or favor. You can’t completely make yourself concentrate, although of course your willingness to concentrate is important. God can remove your ability to concentrate at any time, and for most people, He will.
There isn’t any technique for better concentration that will work all the time. There isn’t any Mass form you can attend that can make you always concentrate perfectly. There isn’t any clothing or prayer technique that can do it. There isn’t any priest’s mode of celebration or musician’s worshipful playing that can do it. That doesn’t mean it’s not important to try; but don’t set yourself up for disappointment by expecting too much. We are not beings of spirit and pure intellect; we are humans.
Thought and concentration are part of what your body does, not your soul, and the brain is a very loopy part of the body. Don’t expect more from “Brother Donkey” than he can give.
In fact… in general, we are designed to react to little changes in our environment, because not noticing this stuff could get us killed. We are also designed to ignore stuff that’s not important at the moment; but unless everyone at Mass acts very similarly all the time, it’s hard for the brain to figure out that what’s going on in the background isn’t important or harmful. Even then, the brain can get freaked that everything is the same as last week, and start looking around for something different. There’s no point getting frustrated that your brain is working correctly; it’s better to stay calm and roll with it. Then your brain might actually buckle down and concentrate more.
So… if you’re not having trouble concentrating, thank God; don’t anticipate trouble; but don’t expect it to last forever. Be grateful when you can concentrate and be grateful when you can’t.