"Beginner" SPD question

[SteveC]

Beginner is in quotes because I've been using them for seven years!
Way back in what ever forum we were using in 2005, I had a problem with my then brand new SPDs.  One of them kept 'locking' very rigidly which felt as if I'd never get out of them in an emergency.  The other one was 'fine'. 
Having now got a new bike, I realise that the pedal which I thought was broken was working properly and the other one never has! 
On the new bike, it's is relatively easy to give the right pedal a firm push down, there is a satisfying click, and the shoe and pedal are firmly joined.
The left foot is much more difficult to get to engage and never does so as firmly as the right.
As this has now happened with two sets of pedals I assume it's the shoes.  Any advice as to what might be causing the problem and what I might do about it?
I keep meaning to get a new pair of shoes but...

S

[Julian]

Try a new pair of cleats?

[SteveC]

I did do that back when the issue first arose to no effect, but seeing they gave me a set with the new pedals I might as well try.

[JonBuoy]

I had to take a Dremel to the tread on my new shoes to give them clearance from the pedal when the cleat was engaged.  They would engage and disengage OK(ish) but definitely didn't float as well as they should.

[Canardly]

Tension screw on the pedal?

[Biggsy]

I'm wondering if it just feels different on the left because the alignment of the cleat on the shoe isn't suiting you as much as it does on the right.  It's not uncommon for something about the rider or the bike to be asymmetrical.  I definitely would experiment with cleat angle and position, and also with the pedal spring retention.  You might need/want the left tighter than the right.  Nevermind the reason.  Just try it.

[Ian H]

I have had touring/mtb shoes where the recess for the spd cleat wasn't large enough to accommodate the pedal in extreme positions. A Stanley knife cured the problem.

[rogerzilla]

You can tell how much the release mechanism is tensioned just by looking at the screw.  There is one on each side of each pedal, and you probably want them all to be the same.