If the unit is more than 12 months old and has not had the battery changed that may be all the problem is.
There may be a problem with the Head or the Stake.
By doing the following simple test you may be able to determine if only part of the unit is faulty
The snake repeller is of course only made up of only 2 parts (the ground probe/stake and the plastic head), it’s unlikely that both parts are not working correctly. Could you please remove a head of a working unit and attach it to the non working ground probe (stake). If it works correctly then the problem is the head, If it does not work the ground probe stake) is the problem.
However, just to be sure, please place the plastic head off the non working unit onto the working unit for confirmation. The result will then determine if the entire unit is faulty or just the head or stake.
Sometimes the problem is the clear cover over the solar panel has got very dirty or discoloured and would be stopping the solar panel from charging the battery properly.
You can then order the required part on line or phone if it is a warranty claim.