Not all cases have the exact same dimensions. I recently had a straight lug case as a project and all my spare bezels had different levels of fit on this case, even my DON's were not all the same. The newest bezels could be pushed on by hand as could one of the DON's (so obviously an unsuitably loose fit), the other DON was very tight and since it was not going on this particular case, I didnt push it to fit. The bezel I did fit, a mid 80's bezel, was very tight and required some filing down of the inside of the bezel in order to fit - this was a cheap bezel so I was ok with doing this.
I'm assuming you are in a similar situation where the bezel is very tight, however I'm not sure that 'one side fitted' can in anyway be correct. The bezel must be fitted using a press and has to remain absolutely horizontal to the case during the fitting process. If the bezel goes down on one side only then it will be very difficult to fit the bezel as the other side will now be at an angle to the case and an even tighter fit, also it could be possible to put a twist in the bezel.
Assuming your watch maker is experienced then he could file the inside of the bezel slightly to get it to fit but the last thing you want is for the bezel to be too loose and pop off unexpectedly in the future.
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