I know this was posted a year ago, but I am having the same problem, and am wondering if you found a solution, Boris Bike?
The pedals were mounted by a specialist so I know that they are on the correct way - the pedals are SPD 520 and the shoes dhb road shoes with stiff base and protruding cleat. Even when standing up I can get the front of the cleat into the pedal, but the back just won't clip in. I think I've undone the tension as far as it will go!
Thanks in advance for any help - I'm new on here.
SPD 520 is an MTB pedal.
Which of the cleats do you have in the pic below?