Uncertainty principle
It was difficult to make equations built for continuous spectra (what you see when you put the sun's light through a prism) fit spectra that just have a few peak frequencies between which there is nothing. Almost everything that had already been learned about light and energy had been done with large things like burning candles or suns, and those large objects all produce continuous spectra. Even though these ordinary-sized things were easy to do experiments with, it had still taken a long time to figure out the law (physics)laws that govern them. Now physicists were dealing with things too small to see, things that did not produce continuous spectra, and were trying to find a way to at least get clues from what they already knew that would help them find the laws of these small and gapped-out light sources.