I also had a similar problem with them and found although you own these addresses you would have to add them individually to be able to send a reply. I couldn't find no instructions to do this but I managed to figure it out by:
Logging in to Gmail via my desktop/computer
Click on settings (gear icon)
Accounts and imports setting
Scroll to section 'send ...
I could not find a straightforward solution myself either because the plus (+) sign is reserved as a special character for "Results that match a word exactly". So I use the following alternative.
A decent alternative
Using the above search terms I can easily find anything but.