"What changed was not my productivity but the nature of the problem"

I think that's the source of the problem: it's impossible to measure the "work" required to solve most software problems. If you tell me to carry a stone up a hill, I can put that in a formula and know exactly how much work I'll have to do. But if you give me a ticket to do a DB upgrade, I can, at best, make an educated guess.

So by the time I close the ticket, how much work have I done, and how do I know whether the time I've spent it proportional to the work I've done?