Solar Panel

Most likely it is the charge controller (what you called: solar regulator ). If this is just a single panel, you can unhook 1 of the wires from the solar cell and check the voltage with a meter. If you are talking multiple solar panels, I would suggest that you hire an electrician to troubleshoot the system (for safety sake).

Not enough sun? The solar charge will only charge the batteries if there is a certain minimum amount of sun on the solar panels which produce voltage (and power), below this level the solar charge will stay disconnected.

Additionally it could be something else simple, that you have undersized the batteries and/or solar panels meaning that the 24 hour average load connected to the solar panel/battery array exceeds the 24 hour average solar panel output + reserve battery storage capacity. What this essentially means is you are drawing more power out of the batteries than the solar panels can put in which eventually completely discharges the batteries.

Did you size your solar panel and battery system so that you had a standard minimum 5 day battery reserve capacity to take into account the variable sunshine hours due to weather and season? Did you take into account other Solar panel output de-rating factors such as ESH (location dependent sunshine hours and output battery charging losses?