Hi Shazz
When I did my first HPT in feb 2006 there was a faint line - then I did a second one 24hrs later and got nothing. Unfortunately for me AF arrived a couple of weeks later.

However when I did my HPT in August 2006 I got a very faint line, 24hrs later nothing, 24hrs later a very faint line again - then I had a BT and got a +ve. So just to say it is difficult to know what it means, if you keep getting BFN and no AF go and get a BT.

Fingers crossed your levels are just very low and you need to wait 24hrs to get that BFP.
T.