There is no shortage of people who will do poorly at IQ tests because of cognitive difficulties and (sure) lack of motivation to practice. Does loading on factors like these make IQ tests less predictive of academic or career outcomes? No, both of those things also require a combination of cognitive ability and motivation.
First tests I remember, I was 9. Then ACT at... 15? Maybe it was 16. Then more IQ tests (3 different ones, IIRC) about 4 years ago. All showed me to be in the same X percentile, with a very small degree of variance between tests and over 30+ years of taking them.
And to repeat, I never spent any time learning how to 'take' a test. It was actually disappointing to get the test results and compare them to ~30 years earlier, and not see any change. I thought I'd have been 'smarter' as an adult, but I do realize it's not really testing that so much as... raw capacity perhaps? That hasn't demonstrably changed, despite all the life that's happened to me. No major health issues tho; I can certainly imagine that had I had a brain injury, my IQ test scores would be lower.
A few years ago I had 3 IQ tests administered to me over several weeks. There wasn't any major difference between any of the scoring/ranking, despite the fact that I was taking multiple. I never got much 'better'.