The SKS key server pool is not a single key server, but a whole pool of servers administrated by different users all of the world. By using DNS round robin, you will be assigned to a different one from time to time (actually, you will get a new set of servers every 60 seconds).
It can happen that a single server is having issues, and there is some delay based on the synchronization schema these servers are using. If a key did not spread through-out the key server network after some minutes or hours, upload it again. If you want to spread a key rather fast, upload it to multiple servers in the network to shorten up the replication paths (and be less dependent on single machines failing or being slow at replicating keys).
Generally, the SKS key server pool is pretty stable and reliable, as (temporarily) broken servers get kicked out of the pool for a given time automatically (and will be re-included as soon as they're working again).