There are a few things that could impact your situation.
The iPad does need less bandwidth that a PC so that is one possibility. This article outlines the bandwidth requirements: Network requirements for SlingPlayer for iPad
This article: Which SlingPlayer software works with my Slingbox give additional information on system and bandwidth requirements.
I'd also check re-download your SlingPlayer plug-in on your PC.
I am actually having the exact same problem, it connects to all my iOS devices, but not my PCs. I called customer support and it fixed for a few days after a factory reset of my 350 device. However now it won't connect through a browser again, I've tried the browser slingplyer on multiple internet connections with very high speeds one had up to 60 mps ( I saw it work during the temporary fix as well) so bandwidth is not the problem. This is very frustrating. No one seems to have an answer.