Pay attention to the editor's notes on your reviews. TextBroker has weird ideas about how commas and semicolons should be used. It's entirely possible to have 100% correct comma and semicolon usage and no other major issues and yet get knocked down to level 3. READ what the editors are saying in your reviews to learn how they want commas and semicolons used. They claim to follow AP Style but this isn't true, strictly speaking. They have a few blog posts where they describe how they want things punctuated, but I would just go straight to the source and carefully read the reviews they leave every now and again.