Every time I try to transfer a third move to my first generation iPad the first one disappears and it's gone any suggestions?

I have a first generation iPad and I always have 9.5 to 10g left in storgage when I try to transfer movies from my Mac Book Pro, it will accept 2 movies fine but when I attemp to move the third one, the oldest (not expired) movie disapears from both devices and cannot be retrived. There are no warnings it just goes. I've asked apple support and all I get are a number of suggested FAQ website referrals which do not apply and an assurence that I can download as many movies as the memory storage space will allow. Has anyone had the problem or an answer.