No upgrade option is available from free membership to paid

Thank you for the plugin. I have some trouble so maybe you can help me out here.

I have created 2 levels and two subscriptions. Once user subscribe to free subscription he should have an option to upgrade it to paid one. The option is there except there is no button to subscribe.

In gateway options I have disabled all choices except paypalexpress but when a user is registered and gets initial free subscription in all members section the gateway shows "admin"

If I manually move this user from admin gateway to paypalexpress, then Subscribe button will appear in the subscription upgrade option.

So my question is what is admin gateway and how users can get paypalexpress gateway automatically when they subscribe.

Thank you,