@Robert7098
You don't give a lot of detail as to what sort of lock this problem lock is, or how you came by it but, if I can just make a couple of general observations.
1/......Stay away from anything online you can't easily return. Although the internet is a great tool, it abounds with products and services that sound too good to be true.....and in many instances are.
2/......When you run a short term rental facility you are dealing with people across all ages and levels of technical ability. The safest option is to keep it simple, that way you minimise the chance of problems happening.
I have been doing this for 8 years and spent most of that time here on he CC and this problem of guest entry not working is a recurring theme. Sure there are smart locks, key-less entry and there are no doubt instances where it works successfully but I can't tell you how many times an arriving guest has said to me...."Ah, thank goodness, a good old fashion key in the lock". They know where they stand!
I have resisted the urge to go high tech, I have a good quality 'Master Locks' deadlock on the cottage entry door which cost me about $25 and I was able to install it myself. I have a lock-box on the wall next to my listing cottage entrance which cost about the same, is self illuminating for night time use.......Robert nothing ever goes wrong with it and every one of my 750+ guests have know exactly how to use it.
So Robert, remember, this is not just something for your convenience, it must be for the guests convenience as well. You obviously either service the cottage yourself between stays or have it serviced by others, you get into a routine after a while with changing the key-code for incoming guests......put this one down to experience and don't try to re-invent the wheel!
Cheers........Rob.