If I have understood everything you have said then the problem cannot be connected with the enlarger. It has to be either the developer( exhausted?) or the paper ( very old MG which has lost all its contrast?)
With simple white light from the lamp and under the lens MG filters then unless the above are the problems you will get different contrasts with different filters, unless of course all the filters are so worn that none can effect the paper.
None of what I have said sounds likely, I know, but I cannot see any other explanations