I have a list with two content types. I've created two custom forms for creating new items (because one field I don't want until the 'edit' form). One works great. The other one doesn't. What's happening is that I have two lookup columns. You can select values for them fine but they don't save when you click 'save'. No errors, warnings or whatevers.
I'm creating these forms in SharePoint designer and associating them with the content types in the list control page. Not sure that it matters but the one that works is the default content type for the list.