duncan,
kg42 has provided some excellent drawings for the cartridge. First, if you increase the pressure to get rid of the backed-out primers all you are doing is hiding a symptom of a problem. It would be best to try to solve the problem.
In the "cartouche maxi" drawing it is the 50.90 length that could help if trying to compare the problem Whelen case to the RWS case that works. Again, in taking those measrements it is not easy to be as precise as might be required to be valid. But, if there are gross differences between the Whelen case and the RWS case then it suggests where the problem may be.