Are you getting a lot of vibration in the tail? I have had this problem before due to an unbalanced tail blade....if you knicked the blade on the ground and unbalanced it it can cause a lot of vibration. If this vibration occurs ic can cause some very strange things to happen like no tail power untill you rev the tail really hard and then have all the power in at once....or if it is really bad you may never even reach that and find that you really don't have much tail power. It is also possible that your tail motor is failing and as it heats up it becomes very weak.