Last year I purchase 4 harbor breez fans with hand remotes from Lowe's. They worked fine after the install.no problem what so ever.I left for the whole summer,no one used the house while I was gone.came back home and only one remote would operate.I reset that remote to the other fan codes,in turn.they all worked.so only the hand held remotes were bad.nothing wrong with the receivers inside the other fans.but why did the remotes go bad?? Lowe's says remote fans always screw up.but why..

Could be the DIP switch contacts. Sliding back and forth a few times may help.
Sounds like you reset the fan switches, then the switches on the remote to match, hard to tell which contacts fail(dirty or corroded?)first. Sooner

Now for some science:

Without a minimum amout of cuurent flowing through the contact, typically 0.010 amps, the contact will oxidize and fail especailly for low level contacts such as the DIP switches.

Reseat them (move them from one position to the other and go through the set up stuff. Power cycle fan or whatever.

Conductive elastomer keyboards, typical for remotes, get junk spilled on them. Not your case.

Batteries die and they also develop an oxide, so removal an re-insertion may fix it.

And if remotes are slid across a rug, they may stop working only to start again months later. Yep, there is a reason for this too and a fix. A non-destructive failure occurs in which time or draining the charge will fix. You remove batteries and with some way discharging, short the battery holder.

Larger cotacts that carry a fair amount of current are not subject to this.

Gold plating helps. Using mercury wetted contactss is necessary in some sophisticated intrumantation.

Normally it's the receivers that go bad. Cleaning the dip switches is a good idea.