Here's what's happened 2 or 3 times over the past 15 years of ownership with my 99 SLT. To begin with how the elec door locks function, at least on the 99 SLT model year; @ 15 mph the CTM signals both doors to lock, when driver stops and gets out the driver door mechanically unlocks, if no passenger gets out the passenger door stays locked. There is no auto unlocking, you'll need to key fob, use key or hit the lock/unlock button on the door to unlock the passenger door. On my truck there is a way to program 2 options on the fob/lock function: 1 lock horn been or 2 lock no beep. I believe on 01 and up there's another option, one press on fob unlock unlocks just driver door, two presses unlocks both driver and passenger doors. Owners manual gives sequences to program CTM/key fob responses.
That said, if the electric function on passenger (or driver's) door is not working or you hear the motor spin and the door doesn't lock/unlock, or sometimes does/doesn't then the issue is with that door's electric lock mechanism. There's 4 options to fix, 1 as Fester mentions, take door apart, remove and clean the electric mechanism and see how long it works, also as Fester mentioned it will most likely fail soon, 2 pay way too much for a new mechanism and replace, 3 see if you can get lucky and find a replacement at a parts yard or 4 get one at a parts yard from another Chrysler line such as a mini van from the same year +/- a year or so. They will not be a direct bolt in replacement. However, the internal parts are the same. What happens that causes the passenger side to fail more often than the driver's is, when driver gets out and fobs the lock, the passenger side is already locked, but the passenger side mechanism still receives signal to lock, eventually the plastic gears in the mechanism wear out. So, with a similar parts yard unit (I've paid $5 at the local u pull it yards over the years) take mechanisms apart and swap damaged gears. This has worked well for me and saved me ridiculous cost to purchase new replacement parts over the years. Hope this helps.
Attention: TDR Forum Junkies 
