My baby won't sleep!!!?

My daughter is 18 weeks and won't sleep in her cot. She falls asleep - usually in her pram - but wakes up as soon as we put her down, even if she's in a deep sleep. We have tried EVERYTHING!! Letting her cry, comforting her, picking her up, but nothing seems to work. This includes when she wakes for her night feed so we are up for around 3 hours every night trying to get her back down. She sleeps ok in her her pram so We've resorted to rocking her to sleep in her pram & leaving her in it for the past few nights, just to get some rest. Please help!!