If you find yourself getting tired, and concentrating is becoming an effort, and you wear glasses, try the following:
Lay your glasses upside down on a flat surface. If one arm is not touching the table, that could be the cause of your problems. It can be especially bad if you wear varifocals.
Do not try to adjust them yourself, unless you're an optician. Go to the opticians you obtained the glasses from, and ask them to check them out. In fact, most opticians will do that for you even if you did not purchase the specs from there.
Ask them to give the lenses a clean while they're at it!
Chances are, that will solve the problem.
One last thing: I'm not an optician or otherwise medically qualified. All of the above is by way of suggestions, not advice.