This isn't a new issue, I've experienced this on the P3D version as well.
When refuelling the 737, waiting for the response to "please use your aircraft to set fuel amount" prompt - sometimes after you enter it, it doesn't work and you keep getting the response repeated.
I have found it usually happens if you enter the fuel amount directly, whereas entering a percentage will normally work.
I almost always have metric units set for fuel.
My theory is when you enter the metric number, PMDG does a conversion to pounds, and then back again, which will result in a slight difference in metric, often very slightly less. Possibly GSX is picking this up as a lower fuel number and thus doesn't think fuel has been set.
Or it could be something completely different.
It's only a small thing - but any way to fix this for good? The work around using percentages seems to work, but it would be nice not to have to do the maths.
Thanks