Based on what the OP has said, it is safe to assume that the filters were secondhand. On the recent Ilford tour I think that Simon Galley hazarded a guess that the filters need replacing every 20 years or so. If the problem is fading of the grade 5 filter, how does he go about ascertaining whether this is the case? Once he establishes whether fading of the grade 5 filter is the problem or not he can then move on to a solution such as replacement or further investigation as to other causes.
I cannot help here as my knowledge is nowhere near good enough but so far I feel that collectively we haven't helped him get much closer to a solution either.