Isla also went through this when she was teething although she was a little older than Riley I think. And then again when she was a toddler and going through fussy eating stages. I was tearing my hair out at the time – I am such a perfectionist and wanted Isla's levels to be in range every single time, and if not I became desperately worried that she would be damaged.
But the one thing I have learnt is that this diet isn't an exact science and it will not always go perfectly. As long as you get it mostly right most of the time it will be fine. Also I made it much worse by worrying so much about Isla – I once tried to feed her formula from a syringe 10mls at a time which made my life unbearable – and hers I should think! I am now much more relaxed and if she doesn't drink all her formula in a day, which still happens every now and then, I just let it go and she almost always has it fine the next day.
The diet is a hard balancing act and it is completely unnatural not to worry but at the same time logically you know it is better for your child if you are relaxed! So I really feel for you. But please be reassured that every child with PKU goes through these phases, whether through teething, illness or just plain stubborness! Isla's levels went up into the 800s once for a few weeks, and she is absolutely fine – in fact one of the brightest (and bossiest!) in her year at pre-school.
I hope things get better for you but please feel free to email if you have any questions.