It could just be a stage she is going through...DD has gone through stages where she would only sleep in my arms (if she went to sleep in her cot it would be up to an hour battling her to get her sleep,and she would be lucky to sleep for 40 mins), and a few times I had to lie down with her on our bed.
I'd just be persisting and hopefully one day it will sink in...lol. Do you have any bedtime routines (for day & night) that you could use to associate going to sleep and the cot?