I assume that by having to isolate the power supply ypu have removed the cover to the alarm panel. If that is the case I would say that the tamper switch in the panel is not being operated.
The tamper switch is usually depressed via a spring when re-instating the cover.
This can be proven by removing the cover and someone carefully depressing the tamper switch. You should here a soft click. Whilst the switch is depressed someone else can try to set/reset the system
Until the tamper issue is sorted the system will respond to any commands
DIY John
September 2010