Ilford MGFB.5k will exhaust most soft working developers quickly. You need a developer based on Hydroquinone Metol to get predictable results. I had some luck with Amidol based developers on smal sized prints.
More likely the build up of Bromide is inhibiting the Metol, this also happens with MQ developers. An example is Ilford ID-20 once sold as a packaged powder MQ developer, initially it was replaced by ID-20 PQ which had far greater capacity. There were complaints about image colour shifting as Bromide built up so it was re-formulatedas ID-62 cutting the Bromide in half and adding Benzotriazole which stops the colour shift. By adding Potassium Carbonate and Hydroxide, instead of Sodium Carbonate, Ilford introduced PQ Universal, which is essentially a concentrated liquid version of ID-62.
When I started printing seriously around 1968 I used Kodak D163, an MQ developer, at that time it was the Kodak Ltd UK & Europe Universal developer, so equivalent of D72. Capacity in terms of the number of prints I could make in a session was relatively low before the developer started to collapse, and I would usually have to make up fresh developer.
Later when I switched to PQ Universal I notice a huge increase in throughput compared to D163. These days I use ID-78 essentially ID-62 with no Benzotriazole and the Bromide increased, but I mix as a liquid concentrate, capacity is really only limited with either developer by take up and carry over with FB papers. Harman Warmtone developer was initially a concentrated liquid version of ID-78, but the MSDS sheets seem to indicate it has been re-formulated.
Ian
