I agree with others that say this has been discussed here many, many times. As always, it boils down to several variables...where you are staying (does that chain even have upgradeable rooms, like Embassy Suites frequently doesn't), or is upgrading a Diamond even a benefit for that brand (about half of Hilton's brands don't have Diamond upgrades as a benefit), and there is not absolute consistency amongst the hotels. Upgrade doesn't always matter to me, but when it does, I ask.
I get upgraded over 50% of the time, my Diamond history is very similar to yours, but I don't think that matters.