The tyre is too tight and we cannot get the tyre fitted over the Stopaflat tube.
Solution 1: Over-inflate the old tube for a day or two before trying to install. That will stretch the tire a little and make installation a bit easier.
Solution 2: Soften the Stopaflat inner tube in hot boiled water (NOT BOILING WATER) for not more than 1 minute. Install it right away after taking out of the water.
The tyre is too loose and there is a gap in between the Stopaflat tube and the tyre.
Solution: Cut-out the valve stem on the old inner tube, and pack that inner tube into the inside of the tire… BEFORE inserting the puncture-proof tube unit. Leaving the old, airless inner tube inside therefore filled-up some of the extra space remaining inside