I agree with Mark that it's likely contamination. Your trays, mixing beakers, tongs, etc must be thoroughly clean of other chemistry before you use them with lith chemistry. It's even preferable to have a tray just for lith printing, simply because of that contamination issue.
I use the same developer, and mix 100ml A to 1,400ml water, 100ml B to 1,400ml water, for 3,000ml developer, and top up with old brown to a full gallon, which is about 3,900ml. Then I use a 16x20" tray to develop my prints. The larger volume insures better consistency print to print, and if I make many prints I replenish with 100ml A and 100ml B after the equivalent of eight 8x10 prints.
At the same time I try to keep my developer warm, at 80-85*F. This shortens development time significantly. Strong lith developer, like you use, should give developing times in the neighborhood of 5 +/-2 minutes at 80 degrees.