This is possibly the most common cause of low voltage. Ensure that there are no trees around and that the solar panels are not blocked by shadow at any time during the day. Keep in mind that a solar systems lasts for more than 25 years andtreesgrow over time. Conducting a bi-annual survey of the installation site is a good idea. .
If shading is not an issue, most likely it will be the higher than normal operating temperature of the solar panels. It has been scientifically proven. .
If the modules are not overheated, the best bet for you will be to check for a bad connection. You can use a multi-meterto check the voltage levels at various points to find out the point beyond which the problem of low voltage. .
It’s uncommon for a solar panel to fail as they’re meant to last 25 years in the field. However nearly all large pv manufacturers have seen product recalls over the past years, and therefore you mean come across failing solar panels.
[pdf] 