CPU’s are great and all but it’s certainly not guaranteed. Your odds have obviously been better but you’re flying out of a major hub or back to one- SFO.
As stated previously CPU upgrades are nice, but this isn’t the real “benefit” GS uses. Its mainly the IRROPS. The ability to throw a GPU on an award ticket is also huge for intl travel/vacations esp savor tickets, but sounds like you’re blowing through all your instruments in the two months you’ve been GS on domestic routes.

What you’re also likely seeing as you slide down the upgrade list or disappearing availability in F are various elite/non-elite buying up at check in- this will always kill a “battlefield upgrade”. You may also be seeing a ticketed higher fare GS or a “protected” GS on a higher fare class. Bottom line here for the non-instrument upgrades, its all about the revenue. The airline will go out of their way on other issues it can like the IRROPS (trust me they move mountains in those) but when it comes to CPU’s and GS down to silver? If they think they can generate cash on it there’s the priority. These routes out of SFO they think they’re going to sell. And if they can’t sell them the the next step is the T-0 buy-up.

Hope you don’t have to see IRROPS or have such a tight connection the Mercedes makes an appearance. But when it happens, it’s a great service.