Could it be you have the wrong prop adaptor.
Some motors have 3 mm shafts, others have 3.17 mm (1/8"). They require the correct size adaptor to work.
A 3.17 adaptor on a 3 mm shaft will be loose and cannot be tightened.
The right adaptor for a motor will be 'snug' fit on the shaft even without tightening the nut.
Although rare, it can be that if you prop adapter fits the shaft correctly you could have a "thin" prop. This happens when you think the hub is tight against the prop but it is actually bottomed out on the adapter itself.
I agree with all of the above. Another possibility, the hole in the prop needs to be slightly larger. If you need to force the prop onto the shaft, make the hole slightly larger. The prop could be preventing the adapter from drawing down on the shaft.
Thankyou guys! I fixed the prop on the shaft. I am going to make a new thread about the difficulties I faced while building the FT Storch. Can't wait for the maiden flight. Hope I don't crash very bad lol. Cheers.