Jumping Back Kick

So I was in class last week and my instructor told me to do a jump back kick, I knew what it was, I've seen many people do it but everytime I try to I can't kl propperly. I would stand in a side stance and try to jump back kick but every time I go to kick my leg just won't extend, it's like my leg is scared or something, it just fails every time I come near to doing it. I can do a normal back kick and spin back kick fine, but it's just this one.

How can I get my leg to extend propperly without failing?

Please help! Thanks to everyone who replies!