[cprocess]

One theory - unless you are replinishing the plating solution, you are plating out the available nickel ions in the solution. As the nickel metal concentration decreases, the plating efficiency decreases as well.
Another thought, make sure you have good solution movement. The solution needs to move around to make sure enough nickel metal is always available at the part to plate. Also, nickel becomes less conductive as the thickness builds, so the solution movement will help overcome the potential resistance at the solution/part interferace.
Also, check the pH of the solution before plating each panel. As you plate, the pH increases so may need adjusting to maintain the same plating efficiency.
The last thought is that since you are plating very thin deposits at this milliamp current, it may be difficult to get an accurate thickness reading with a micrometer caliper.

Hope this helps. If not, maybe it will give some insight on topic to research for the science project, thus better understanding how nickel plating works.