Store Front Glass Door is Hard to Lock and Unlock

If your key is working fine when the door is open but is very hard to turn when locking or unlocking the door, then the problem is not in the lock. There are three things to check for.

1. First, check the threshold and make sure there are not any small rocks or other debris that are being squeezed by the door. When debris is present, you have to push the door in order to get the bolt to lock AND push the door to get it to unlock. What you are doing is relieving the pressure from the bolt enabling it to operate. Remove any debris and repair any screws that may have risen in the the threshold.
2. Second, if your door or door frame has settled, then the bolt receptacle in the jamb and the bolt are not lining up. Specifically, the bolt is too high and is stopping prematurely by hitting the top of the jamb cut out. Yes, it will lock when it is like this, but you will notice by using a flashlight that the bolt is hitting metal before it goes through the complete cycle. Take a small round metal file which you can get at Lowe's and file to top of the cut out and give the bolt another quarter inch within which to move. 
3. Third, if the door is warped, you have to push in on the door in order for the bolt to engage. Using a metal file, file the part of the jamb bolt slot nearest you on the outside enlarging the hole.