There are many factors at play when it comes to your first solo.
1 - Same instructor throughout?
2 - Weather?
3 - Your performance on each lesson? Everyone has good days and bad days, so did I. If you had a bad day and make a couple of mistakes, it will take another 2-3 circuits for the FI to be convinced that you've fixed it. If you do it again, then it's another 2-3 circuits at least.
4 - How often are you flying?
etc......
How old are you if you don't mind me asking? This can have an effect as well.
I'm 29 and I recently just went solo at 17.3 hours. Even at 17.3 hours I was getting frustrated with my progress at various stages.
In hindsight it didn't matter how long it takes to go solo. Don't chase the numbers so you can brag about it.
Having said the above 40 hours is quite a lot. Rough average is around 20. Perhaps it's time to seek a second opinion from another FI?