This is common with old freewheels. Rust, dirt or dried up grease make the pawls too sticky for the light springs and they stay in the retracted position. Also sometimes dried up grease can fill the ramps so the pawls can ski over them rather than engaging.

Most respond very well to a long soak in a bucket of solvent (naphtha or mineral spirits) followed by some agitation to flush out the crap. then drain, dry and re-oil by running some sticky oil through the gap until the excess comes out the opposite side. Drain again for a while so any more excess won't spin out all over the wheel and you're good to go 95% of the time.