If you have a loose engine, it can and will move around. Each landing would reposition the engine.

Gordon has broken the code.....On certain mount straps, the shim is required under them to make them tighten sufficiently.
If no shim material is included with your engine mount straps, just slip a thin metal shim between the strap and the engine can to make the straps effectively longer. The local hobby shop that carries K&S metal will have some thin brass or stainless steel shim that will do the job.
Don't overtighten them....it is possible to deform the engine case.