Antique Kenbar Sewing Machine Skipping Stitches?

I do wonder if anyone out there can help on this one. Why has my grandmother's 80 year old hand-crank 'Kenbar' sewing machine suddenly started 'skipping' stitches and puckering up the material? It's been an absolute treasure until now!

I was just about to source new needles for it, but that won't be worth doing if it has finally died on me!

By CK