Hi There,
There can be two reasons for this
1. On form customization after clicking on save button, you have not clicked published
Fact: in order to make the changes available to users, you are supposed to first save and then publish the form/customization.
2.On the left navigation, just above the general tab; is a form by default. it is possible that you have made changes to some other lead form and thats why its not visible. hope you have made changes on the default form and looking at default form only.
fact: there can be two lead forms in the system main form and mobile form; after update rollup 12 it can be upto three.
You can go through this
blog.incworx.com/.../how-to-customize-a-microsoft-dynamics-crm-2011-form
Hope this helps !!