You'll fizzle more as you level up if your relevant casting skills don't increase at the same rate. I don't know if there's some extra breakpoint at 40, but I expect it's more a matter of perception. 100 divination is around max at level 20, but it's only half of your max at level 40. If you've leveled too fast for casting skills to follow, which is easy with 10x XP and skills like abjuration that don't get used constantly, then that's why you fizzle more the higher level you get. Especially if you then try to use higher-level spells that you gain as you level.
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