Sounds like a fun project! Would love to see more as you progress.
As for the thrust issues, either your scale is not very accurate but for such small motors it is rather hard. Another option is to use pivot so that as the motor pushes sideways the pivot will press onto the scales.
It might seem silly but are you sure you have the correct props and power for the motors, also are the props on the correct way?
Could you let me know where you found the claimed thrust values of I can have a look and see?