The first thing you need to do is try to figure out what software your clients are using that's blocking your site. (Or maybe it's their ISPs that are using something.) Yes, there are lots of possibilities, but the first one to start with is probably Websense. Even if Websense isn't responsible for your clients' problems, it's worth seeing how your site is categorized.
Websense has a "site lookup" that you can use to see how they categorize your site, but you have to set up an account and log in to access it:
From Websense's contact page you can (without registering), check how they categorize your site with SurfControl (another filtering product that Websense bought out) and you can request that they re-categorize your site if it's wrong: