Trouble Printing Envelopes using TS6420

[Stephori]

When I attempt to print a #10 envelope,  a message occurs saying I don't have a #10 envelope in the rear tray, BUT I do.

Following the instruction to print on whatever paper is in the rear tray, the envelope is printed, but the flap is on the wrong side.

[normadel]

So, turn the envelope around next time.

[Stephori]

I tried that, but then hardly any of the address appeared on the envelope.

[normadel]

You first said the flap is on the wrong side. Then it's the wrong position. Can you provide a photo showing what it's doing?