Re-recording ancient reel to reel tape?

My little Aiwa 1/4" tape recorder sat in a drawer for decades. Tape recorded 30 years ago plays at normal volume but now will only record at an extremely low level. I can hear the playback, but only barely. The tape stock is 40 years old. It erases fine. The volume/level control seems ok. Does old tape lose its ability to record? Do microphones dry out? Suggestions?
Added wrinkle: with the recorder set to record, tape rolling or paused, and the earphone plugged in, the feed through the microphone sounds fine and responds to level adjustments. There's still barely any level on the tape. Help!
Maybe the bias circuit in the recorder is out.

Could well be a head clog on your record head. Clean carefully. Try again.