I had this problem with the GH New York. The response varies according to how much the GP rep is willing to help. If they just check their system and say no, they're not going far enough.

In MY experience the reps that put you on hold and call the hotel directly, leaving you on hold for 10-15 mins are the ones who are trying their hardest.

Either keep trying, and ask the rep to check with the hotel directly, or call the property and ask to speak to in hous reservations. I do however think it is unreasonable to expect a higher category suite than what is availble, though the GH New York regularly allows Diamond members to book into higher suites. It's a great benefit when it happens, but shouldn't be expected.

You will find, like airlines, hotels will oversell. It's very easy to upgrade someone to a 2 bedroom suite when you have oversold the one bedrooms. Seeing it as available online, doesn't necessarily mean that there is one vacant.

Even if it's not available I say it still kicks SPG's a$$ where the platinum suite upgrade is so inconsistent, and in my experience, is rarely honoured by the properties where you want it the most.