As I said before, there was recently a bad batch of bulbs, and I bet that is the problem. Your reptile shop should replace it free of charge. You can bring the bulb and fixture to them for testing if you like. Did you check the fixture with a different bulb, like an LED?I’ve attached a photo of the bulb, the fixture and what the inside of the fixture now looks like. Thanks
Also, you don't want to use a "spot" bulb. Spot bulbs concentrate too much desiccating IRA into too small an area and contribute to pyramiding. Please read the link that I left for you in post #13. It will explain a lot of this. At the end is a complete breakdown of the four elements tortoise lighting and heating.