What about CPU temperatures when the CPU is under partial or high load?
Do CPU clocks under load or a benchmark performance indicate thermal throttling?
Does the exhausted air remain relatively cool?
Is there an easily noticeable temperature difference between the two tubes of the AIO?
Especially if the answer to the last question is yes, then a likely cause is that the channels of the waterblock's cold plate are clogged with dirt. (Also, since you say it happened suddenly, I consider this more likely to be the cause, rather than a bad mounting.)
If so, and if the AIO is no longer under warranty, you could take it apart, clean the cold plate, rinse the radiator and tubes, refill, and close it up again. Now the last two parts of this are going to be hard, unless you add a small reservoir into the loop.
Obviously, before you look into whether dirt in the loop is really the cause, you might as well reseat the cooler and see if this already helps. While you do so, check how good the previous TIM imprint was, clean the old TIM off, and try to achieve the thinnest possible but complete TIM coverage, of course.