Steps to Reproduce:

1. equipt elytras (unbreaking elytras have better chance to reproduce the bug)
2. take some rockets
3. press space/jump and right click/use the rockets

Observed Results:
Sometimes, although the rocket seems to be used, you wont take off. You will observe yourself being slighly "pulled" up but not really tacking off. If you press space/jump once or twice right after the rocket failed, you can still gain your boost back and finally take off.

Expected Results:
When you jump and use the rocket, you should be boosted to the air or the direction you are facing and take off completly. 

Notes: Taking off is much more dificult on mobile. A change similar to the one done to Java edition that gives more time to use the rocket when activating elytras, might help the problem or even minimize the bug's consequences