It is likely that the variable line spacer mechanism (controlled by the button on the end) is gummed up. It is common, and can usually be resolved by pulling the platen and putting a drop of sewing machine oil on the parts that slide in and out which engage and disengage the ratchet, circled here:

The instructions on how to remove the platen can be found in the 1945 Ames Typewriter Mechanical Training Manual manual for standards, the pdf of which is posted at the bottom of the page here.