My vitamin D levels are not increasing?

It's best really to take vitamin d that's carried in oil. Tablets are the least well absorbed form. Because it's a fat soluble vitamin it's probably best taken with food or a meal containing FAT.
For sun exposure it's best to expose skin that is normally kept covered by clothing. Tanned skin makes less vitamin d than palest skin AND Sunlight contains both UVA and UVB rays. The UVB converts 7 dehydrocholesterol molecule (a form of cholesterol) into Cholecalciferol. (vitamin D3) however further UVA exposure converts that newly made vitamin d3 into suprasterols the body doesn't use. So exposing skin normally hidden enables most vitamin d to be created and covering up that skin for the rest of the day PROTECTS it from degradation by UVA and allows time for it be be absorbed into the body.

Generally people need 1000iu for each 25lbs they weigh and I guess you probably weigh more than 50lbs. So try increasing the amount you take every day until you have another 25(OH)D proving you have reached your target.

No your results are fairly typical. If you find the GRASSROOTSHEALTH website you'll find a graph of different daily intakes and the vitamin d levels they find needed. While their is a big variation for the lowest to the highest responders to get EVERYONE above 40ng/ml probably would require over 9000iu/daily each. It's not surprising because we know that generally FULL BODY NON BURNING SUN EXPOSURE creates 10,000iu ~20,000iu so I don't really understand why people think using trivial amounts of vitamin d would solve the problem.