Your UVB bulb is probably dead and still turning on

This one catches conscientious owners, which is exactly what makes it worth posting. UVB bulbs stop producing usable UVB long before they stop producing visible light. The bulb looks identical. It switches on, the enclosure is bright, everything seems fine, and the animal has quietly been getting nothing for months. The rough replacement schedule manufacturers give: T5 tubes about every 12 months, T8 tubes about every 6, mercury vapor somewhere around 6 to 9, and the compact coil bulbs are shortest at roughly 3 to 6. The entire system for staying on top of this is writing the install date on the bulb in sharpie the day you put it in. Without UVB a reptile can't make vitamin D3, without D3 it can't use dietary calcium, and the end of that road is metabolic bone disease. Rubbery jaw, bowed legs, tremors and twitching, kinked spine, lethargy, fractures from nothing much. Early MBD is largely reversible. Advanced MBD leaves permanent deformity. It's one of the most common things exotic vets see in captive reptiles and it's a husbandry problem almost every time, not bad luck. Two other things that quietly kill your UVB. Glass and most plastics block it, so a bulb sitting on a glass lid is doing very little for the animal underneath. It wants to be over mesh, or mounted inside the enclosure. And distance matters a lot since intensity drops off fast, so the same bulb hung too high is effectively a weaker bulb. Check what the manufacturer specifies for distance and mesh, because it varies by product. If you'd rather know than guess, a Solarmeter 6.5 reads UVB output directly. Not cheap, but if you keep several animals it pays for itself in the bulbs you didn't need to replace on schedule and the one you didn't realize was dead.