help with key fob please

[woodhead]

Hi. Having trouble programming key fob on mk6 transit.
Tried programming one to my transit but couldn't get it to work, so tried one from my other transit and that worked fine, then tried one from the 1st transit on the 2nd but that one wont work. Then tried another key fob on the 1st transit and that one wont work.
So now i have one fob that works on both transits, one that only works on one, and two that wont work either, both the ones that dont work are 2nd hand so could be faulty, but why wont the one from the 1st transit work on the 2nd.
Hope this isn't too confusing and any help would be much appreciated.
Thanks Mick