Post at least a diagram of exactly what is connected where and how, or take some photos if that's not possible for any reason. We don't have enough information to give you any insight whatsoever at this time
Hi,
If i understand you right, when the output of the panel is lower the buck does not work but when the panel output voltage is a little higher the buck circuit works ok.
If that is true then i would guess that you are working too close to the max power point of the panel/array.
The "max power point" is the operating point of the solar array that provides the maximum power possible. It varies with insolation level so as the sun goes down the max power point moves and it then indicates that the max power you can get is less than with full sunlight.
Now before the max power point is reached the voltage output would be somewhat constant with any current within the range of the array for that max power point, but unfortunately once you draw enough current to get to the max power point the voltage drops off fairly fast. So you can have 25v out at say 100ma and then as you go to 110ma the voltage drops down to 15v or even less. Also, if you have 25v out at say 100ma and the insolation level decreases, the max power point moves to a point of less current and so now you draw 100ma but the voltage falls down lower than 25v because now the current is beyond the max power point.
There are circuits that help you maintain the max power point for any insolation level but if your circuit needs 100ma and the max power point is at a point where the current is only 50ma, the voltage still falls very low.
If that happens on a regular basis, then you need a larger solar array.
It does sound like you need a larger array unless you can limit the use of the circuit to times when the sun is brightest.