Is it possible for you to scan and show us the examples?
In my experience, once the image begins to form in the developer, you can double that time to figure out when full blacks will appear. That is, if it takes around 4 minutes for the image to begin to appear, then I will probably be happy with the blacks by the 8-minute mark. This is just a generalization, as a lot depends on the print and paper, but having developed around 500 sheets (of mostly Foma paper) in Moersch EasyLith and SE5 lith, this is what I've noticed. Do you remember when you first noticed the development start? The only time I had something like what you described happen was using Fuji's Fujibro paper (30-60 min of development) at 'normal' developer temperatures (around 20 C). When I used hot lith (over 30 C) the development times were reduced to 5-10 minutes. Try keeping your dilution the same next time, but increase the temperature and see if that helps.