Might of overheated?
When reassembled you want the transistors pressing firmly against the heatsink, that paste although much better than air does not conduct heat at all well, it's designed to be used with a thickness of something like 0.01mm.
Also, make sure you and the soldering iron are earthed when handling the transistors, while not connected to the circuit board they might be sensitive to static which can cause them to fail long after installation.
When reassembled you want the transistors pressing firmly against the heatsink, that paste although much better than air does not conduct heat at all well, it's designed to be used with a thickness of something like 0.01mm.
Also, make sure you and the soldering iron are earthed when handling the transistors, while not connected to the circuit board they might be sensitive to static which can cause them to fail long after installation.
