Fobs do not spontaneously un-program themselves; it takes very specific steps, which take a half-hour, to make a known fob unrecognizable. Adding a new fob is a simple matter, but not applicable in this case. If the fob works in the slot, it is programmed; it will work with a low/dead battery, or even with no battery installed in fob. Trying to reprogram the existing fobs will only muddy the waters, making getting to the bottom of the problem more difficult.The easy solution is to have the two fobs reinstalled as fob 3 and 4. My wife washed and dried a fob. Rather than program the new one back into the fob 1 position, it was easier to make the new one fob 3. I believe there are 8 fobs that can be installed.
That was Chevy's solution.
There are usually only a few reasons why a fob may not be recognized: A low fob battery, low car battery, interference, or a combo of two or more of those things.