I've had the occasional hesitation due to a peachy one but never a failure to deploy automatically. The logic of the system seems to have been designed to reduce the chances of this happening to a minimum.
Extract directly from the 737 manual:-
During landing, the auto speed brake system operates when these conditions
occur:
• SPEED BRAKE lever is in the ARMED position
• SPEED BRAKE ARMED light is illuminated
• radio altitude is less than 10 feet
• landing gear strut compresses on touchdown
Note: Compression of any landing gear strut enables the flight spoilers to
deploy. Compression of the right main landing gear strut enables the
ground spoilers to deploy.
• both thrust levers are retarded to IDLE
• main landing gear wheels spin up (more than 60 kts).
The SPEED BRAKE lever automatically moves to the UP position and the
spoilers deploy.