I am not so familiar with enzymatic kits for quantifying glutamine, but most enzyme-based analysis kits come with a number of fairly strong assumptions on the enzymology involved, and in my experience they are difficult to use reliably. Commercial kits are hopeless to troubleshoot since their components are not disclosed --- you don't know what you're working with.
Are you at a clinical chemistry facility? If so you may have access to analyzers used to measure blood biochemistry parameters, and some models measure glutamine, for example this one. These instruments are also based on enzymatic assays, but they are carefully tested since they are used in human medicine, and so I would consider them more reliable than the average Sigma kit.
The best alternative I think is mass spectrometry, or NMR. Glutamine is easy to measure on any LC-MS or GC-MS setup (an old triple-quadrupole will do nicely), it ionizes well in positive mode electrospray, is well retained on a typical HILIC column, and isotope-labeled standards are readily available. I'm no expert in NMR but I know glutamine is measured routinely.
Another caveat with analyzing glutamine is that it decomposes spontaneously in water to form pyroglutamate, with a half life of about two weeks at room temperature, as shown in this paper. But I'm sure you're aware of that.