Well, either way, it would be up to you to fix it. We wouldn't have anything to do with the server (well, unless you were hosted with my company, but the PHP project itself isn't any way related to the corporate stuff). Of course, it could just be the ambiguity of the term "you" in the sentence throwing me off here.
Lastly, if you suspect that it is the load-balancing, and the balancer isn't capable of persistence itself (such as if you're using round-robin), and sessions themselves are breaking, it's probably because you're relying on file-based sessions, which do not (by default) synchronize between servers. Instead, you'll need to centralize your sessions in a database, memcached, or similar option.
For some hints on session management and how you can manage it across server clusters, check out the session_set_save_handler() function http://php.net/session_set_save_handler