This told me that the problem was not her key but the pins in the front door’s lock, and boy howdy were those pins worn! Some of them actually had a concavity from keys sawing through them over the years. If your most-used lock doesn’t work well with your key but works well in other locks then the pins may be worn down. Have someone rekey the troubled lock to match whichever of your keys that works the best. For high traffic locks consider asking the locksmith to use nickel silver pins. Nickel silver is a harder alloy than brass and will last longer (though it will wear down brass keys faster!)
I rekeyed the lock to her key and it worked great. Who knows if it will work well for everybody else though. They are all used to rattling their keys around in the lock and there are variations in all of their hardware store duplicates. Some of the residents might notice their key works better after the lock is changed and others worse. For this situation it is a good idea to have extra keys on hand for those whose keys no longer work. Sell them copies of your key which is now essentially a first generation key.
If you have a little extra money ask the locksmith to code cut the original key and save it for making duplicates.