pmatil, mirabilis in general seem to have very large roots systems so that is certainly a possibility as for your plant's down turn. As I mentioned, mine took a long time to settle in, but it has been doing very well for a year or more. I am almost afraid to move it at all for fear that it will pout again. For now, at least, I am really enjoying its unique pitchers.....
My Mirabilis var. Ech. started making pitchers almost instantly I got it bare-root. But over time the leaf size has gotten smaller (first it got bigger real fast) and I don't know why. Conditions are the same but I suspect that the roots have overtaken the pot. This one seems to have extensive root system.