3g keeps dropping

Both my girlfriend and I had Iphone 4s's at the same time (Sept 2012) and both on 02. I've always had poor 3g reception at home but only last night I noticed that she had solid 3g continuously (also the 3g was in blue) Mine on the other hand will just have the little white dot at the top of the screen. If I turn the mobile data on and off it sometimes gets 3g (in white not blue) but then will only hold it for a few minutes before going back to the little white dot. Additionally this only happens at home. In other areas it picks up 3g and wifi no problem.

This may not seem much of a problem but I don't have broadband at home (only 0.5gb available!) so this is only way I can access the web if I need something urgently.

I've tried:

3g on and off

power phone down

sim out and in

network settings reset

o2 have said the 3g coverage at home is variable.

So, here are my questions:

At home, why is she getting continuous 3g (in blue) and I'm only getting a few minutes of 3g (in white)?

Is there anything I can try?

Do you think my phone is faulty?