Check wire connections and fuses:
1) Check whether the screw terminals are loosen up or not due to corrosion, freezing, thunderstorm or even during transportation. If screw terminals are loose, tighten the screw terminals.
2) Check whether there's are any reverse polarity on connection or not. If reverse polarity persists, rectify it ASAP.
3) Using a voltmeter check terminals and wire levels.
4) If the fuse, wires or terminals are blown or damaged replace with new ones.

Check whether direction of solar panel towards the sun or not and solar panel is performing ok or not:

1) Look for burned out solar cells.

2) Check whether the junction box is ok or not. Also Junction box's wires are loosen and corroded or not. If the junction box is not ok or damaged, replace it with newer ones.

3) If there's shade over the solar panel, make arrangements to get rid of the shades. Because the slightest of shade over a solar cell can degrade the performance of whole system.

4) With the help of the multi-meter, try to measure the open circuit voltage of each individual solar panel and compare the data with the manufacturer’s datasheet for your PV module to help confirm performance is as designed.

Check battery performance

1) Check whether the batteries are damaged, leaked or not. If it is, then replace the battery at the earliest.

2) Check whether the discharge time, charging time operating properly as per the specification or not.

3) Verify whether all the interconnection between the batteries are tight and corrosion free.

4) Check the battery specifications, whether extreme weather is affecting the performance of the battery or not.

Recheck design parameters

1)Check worst case sun hours for your region and compare with the design parameter, because this extreme weather condition can severely affect the performance of the system.

2) Check battery capacity during the cloudy, rainy or without sunny days and compare with the design parameters.

3) Review whether the tilt angle is ideal for whole year with consideration to Azimuth.

4) If inverter is used in the system, check with the manufacturer whether the inverter's efficiency or energy consumption factor was calculated or not.

Check appropriate ventilation to prevent overheating

Electrical equipments like solar charge controller, inverters can overheat and temporarily shut down. To address overheating:

1) Follow the user manuals guideline to install correctly the mounting brackets and maintain proper spacing and ventilation.
2) Clean dust, debris dirt that interferes with the ventilation.
3) Assess whether the spacing and ventilation in enclosures are appropriate.