Thank you for using the Support Forum. Please check that you have Auto Continue in the System Setup Menu set to "On" and make sure that the Paper Source on the Paper Menu is set to Auto Select. Then the machine should pull the closet size. I hope this helps.
I have similar problem with my 3600N. I have the paper set to Plain Paper on the menu and have loaded plain paper in Tray 2. Every time I try to print, the message comes up "Load Tray 2 Letter...". The printer will finally print after I wait approximately 30 seconds for each print job. Is there any way to shorten this time? Or another option to substitute paper from Letter to A4?