The firmware on all stack members need to match.  If the versions do not match, then the ports on the member switch will not become valid for operation. This condition is known as the Suspended Stacking Mode. Some models will synchronize the firmware on the member switch with the firmware that is running on the Master Switch. What is the model of the switch you are stacking? It could also be a hardware failure on the switch.  If the switch is covered under warranty you may want to contact support by phone.  1 800-945-3355  Hope this helps,
Hello,  I have four Dell power connect switches assembled in a stack. Three of them are able to join in the stack but the fourth one o continously fails. i have tried to swap the cards, swap the cables, restore to factory defaults, format the switch using option13 but all has failed. The switch works just fine as a stand alone but fails to join the stack. I really need this switch to be in the stack. any ideas. Thanks. 
Failing to join stack