SIMM sockets may develop solder fractures or internal breaks that will cause problems, either when the machine warms up or when it tries to boot. Resoldering may help, or it may require replacement of the socket.
The plastic-tabbed SIMM sockets used in the A4000 are fragile, and if
the tab breaks off, you'll need to either replace the socket or come up
with some other mechanical method to hold the SIMM firmly into the socket.