DC-DC & Solar Question
The power output of a solar panel depends critically on the output voltage and current. There is an optimum point, called the maximum power point. You could say it is the "sweet spot" for the panel.

If you try to draw more than the peak power current from the panel, the voltage will start to drop fairly rapidly. So, assuming the regulator works, I think you simply tried to draw more current than the panel could supply, thus causing the voltage to collapse. If the panel was not in a good sunny area and pointing directly at the sun, then you could try that to see if it helps.

There is nothing wrong with using a DC-DC converter, but you have to make sure it does not try to draw more current than the panel can supply. There may be some DC-DC converters which sense the maximum power point and scale back on output current to keep the panel in that region. You can google MPPT converter. See what you find.