Cantilever rear brake ineffective Front brake works fine.  Rear brake does not. 

I took the bike to the LBS to have the rear brake adjusted and get new pads. I expected them to work great, BUT, brake is still not stopping the wheel as it should.  Bike shop guy said something about the pads have a "glaze" on them that has to be broken in by using them.

What other possible causes should I investigate ?
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