Cantilever rear brake ineffective
Easy things to check:


Pad not aligned  with the rim properly
Pads too far away from rim, requiring excessive lever travel before engagement
Excessive toe-out


You may also have too much flexibility in your brake cable and housing which limits the amount of braking force at the lever which is translated to the calipers. There's no obvious way to tell if this is happening, but if the housing and cable are old it's not expensive or difficult to replace them.

There a good park tool page on Canti brake adjustment, you can use this to see what the proper adjustment looks like.

https://www.parktool.com/blog/repair-help/cantilever-threaded-post-brake-service