Oh, come now, those are the terms of the program but frequently not how things operate in practice.

1. Most hotels do preblock Platinums.
2. Some upgrades are held back, assigned at checkin perhaps, in which case being early could help.
3. Some upgrade rooms may be held back, hoping to sell or to handle oversells, and checking in late may help (without status a late checkin is often helpful)
4. We know that under the T&C the room has to be available 'throughout the stay' and thus a short stay is helpful as it ties up the room fewer nights. If deciding which plat to upgrade, some hotels will likely give the upgrade to the shorter stay all else equal.
5. Just because another Plat is preblocked into a suite, sometimes the guest that isn't so preblocked can push... get the suite that had already been set aside for someone else... or get assigned to a specialty suite technically not eligible for upgrade.

When an upgrade really matters it's best to
* know the property and its history, how does it usually treat plats?
* know that rooms it generally upgrades into
* know how available those rooms are
* consider contacting the property directly a few days in advance, my own perference is by fax