After each assembly step, before initial power-up and whenever you experience some problem, a thorough visual inspection will often solve 50% of the problems.

Use magnification and good lighting
Make sure you have good lighting and use some type of inspection loupe or, ideally, an inspection microscope (if you are so lucky) and carefully and methodically inspect every connection on the board – paying close attention to any adjacent solder joints (bridging).

Solder Joints
There can be many variations in a solder joint – some are mainly aesthetic (too much/little solder), some are ‘time bombs’ (cold joints) which can work for a while, then at some future date, just ‘crap out’.  Here is an excellent overview of solder joint issues by adafruit.

Solder Bridges
An unintentional ‘blob’ of solder that ‘bridges’ (connects) two pads that are geographically close together.  Most often these can be repaired with a quick touch of a cleaned soldering iron.  Egregious bridges may require a solder sucker or solder wick.

Screw Terminals
It is possible to “torque the hell” out of one of the screws and break the associated solder connection.  Look closely (use magnification)