HP Deskjet and Envelopes

Have used the HP 870cxi for years with no issue. I now have a need to print on envelopes and have problems. There is a device that accepts the envelope but this device can be slid to the far left or far right. When on the left side printing leads to a flashing yellow light. When on the right side the envelope goes through as if it were printing but no type is seen. The ink cartridges are current and standard paper prints well. Advice sought