First time poster so please excuse any mistakes I might make

Ive now done two brews, and have been brewing with a somewhat experienced brewer so Ive been kinda trained but not a lot.
My first brew was a coopers aussie pale ale, and my recent brew is a coopers real ale kit.
Both times Ive fermented for a week, then racked it to a standard keg with s30 valve and left in the same warm place for a further week before transfering to a very cold shed. Both brews although very tasty (especially after the addition of some steeped fuggles and dry hopped cascade after 4 days) have lacked any real fizz. Yes, theyre slightly carbonated, and obviously we dont want fizzy ales, but there is no real head to speak of, and what is there is gone pretty quick. Now my two brewing compadres have used similar barrels and kits, and have lovely creamy heads and great body.
From reading a few threads on here Im thinking it might be due to the fact that Ive always gone straight from my mates warm brew room to my cold shed, and therefore they havent had time to fully carbonate, is this the likely cause? I have been and bought an s30 cylinder and have been gassing every night int he hope that i could force some carbonation through, but so far ive had no joy.
Its been outside now for a week, will bringing it back inside have any effect or is this brew doomed?
Thanks in advance guys
matt