I believe there will be a simple flaw in my understanding somewhere.

From what I can tell for any given block you increment a nonce up to 2^32 (4294967296) per second (and the timestamp every second) in order to find a suitable hash.

Thus if you have a mining rig that can do 4.3Ghash/s, you can cover all possible block hashes in any given second.

What am I missing here, and if this is the case, why do mining rigs with this specification or higher not find all the new blocks?