In Spider, Classic DO8 or in Combi36 modules might be a situation when relay #8 becomes irresponsible on commands while relays #1..#7 are working fine
If manual switch control of the relay is working fine, the most possible reason is that correponding DO point at the FX PLC is set to "watchdog" which may be used for alarm issuing.
To avoid the situation, you must check if the corresponding DO module (Spider, DO8 or Combi36 DO) is configured with "watchdog delay" as 0 sec.
Go to "Programming" -> "Modules" and select corresponding module:
If delay is set to other value, you might need to check if "watchdog" feature is used in this system. If it is not used, setting delay to 0 sec will return relay #8 operation to normal.