Are you using the big propellor?
Try to airtube thing and if that doesn't work then try shaving the big propellor down a shade with some sandpaper or a Dremel.
A lot of people have issue with the big propellor and a 1200 because that is the absolute limit what the mod is capable of handling.
If you are still having issues, then put in the small propellor. You don't lose all that much flow and it's far more reliable.
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