If it's any consolation, I rarely get upgrades as a Diamond. I've actually only ever had one, and it was on a Fairmont Gold floor, from a base Gold room to a higher category one, for a single-night stay.

I'm not sure how accurate it is to do an availability search to see if an upgrade should be available. It's possible that a hotel may still be advertising rooms for sale, even when none are available in that category, on the expectation that a certain percentage of guests will not show up, no different than an airline selling more seats than are available. And even if they have availability, the problem is you have no idea how many other Platinum or Diamond guests there may be, also looking for that upgrade. It's just not a transparent process, and a hotel might be operating fairly, or might be completely screwing you out of an upgrade you are entitled to, and you have know way of knowing or proving it. At least many airlines publish the upgrade list, the whole flight closes at the same time, and you can see exactly how many seats get filled and by whom and where you stand in line. With a hotel, the better rooms could all be sitting completely vacant and you would never know. You don't know how they select guests for upgrade, if it's based on the order they check-in, or if they pre-allocate the upgrade based on status or revenue history or something else, or if it's just based on whatever is available in realtime to the front desk agent you happen to be speaking with. The whole process is as clear as mud.