This can happen. In this case, take the complete wheel and, while spinning it, bounce it on the ground a few times. This procedure usually remedies the situation and moves the tire properly into its rim bed. If this has no effect, it is advisable to mount the wheel and drive a few meters, this will then push the tyre into position in any case.
During mounting, especially in the paddock, you must make sure that no mud, soil or any foreign matter gets between the mousse and the tyre. Such dirt acts like sandpaper and significantly reduces the life of the mousse.