It could just be that the platties suffered more than your other fish, and therefore are taking longer to recover from the toxcity the low pH caused them. There's a 50/50 chance the platties won't make it depending upon how severe the damage was done. Rectifying the pH and bounching the values around too much too quickly can also hurt fish very much as well, and put an already stressed fish over the edge.
Platies like a high pH tank, kinda a little brackish, but not as brackish as mollies. If your pH dropped really really low, then the platy physiology would've suffered much more than the tetras, who like lower pHs to begin with even though they can survive higher with proper acclimation.
The only way to keep pH stable is to do routine water changes. A tank that has not been water changed in a long time will decrease in pH slowly due to build up of acids in the tank (carbonic, ammonia etc). As the system slowly degreagates, the fish acclimate, but they all have a threashold on how low a pH they can handle, and begin to suffer at that point.
~~Colesea
