I can't reset my service lights
 

Hi I wonder if someone could give me a bit of advice..
What it is I always do my own servicing on most of my cars and recently have just given all 3 BMWs in the family a service..

I've done the 19 and 7 service reset trick on my z3 and on my fathers compact a few times now without any problems but on my 1997 1800 ti Compact it won't work for some strange reason any suggestions please on the reason why.