The fuse clips and the fuse metalic ends oxidize over time. The oxidation is resistive and when current passes through it, heat is generated. If you keep the fuse clips polished and bright and shiny, the contact resistance is minimized as well as the heating. The fuse itself will heat some when current nearing it's capacity is passed through it. Added fuse clip heating can melt the plastic.
Problem is, once the clips heats enough to melt the plastic, the clips embed themselves into it, and they are no longer able to float and align themselves so they make full conact along the width of the fuse contact. Forcing currents through a tiny path also creates high heat. Just like using too small a wire would be. I've seen fuse blocks get hot enough to melt the solder on the wires attached to the back of the fuse block. While the wire still makes contact with the connecting lug, again the contact area is reduced and heating occurs at the narrowed current path. Depending on the severity of the melted area, the fuse clips may be restored to floating/ self aligning condition, the clips may be polished and all oxidation removed, and the backside wire resoldered (carefully not the melt the plastic further) and the block can then be restored to proper operation. Or, you can rewire the block so one of the spare positions become the main (polish the clips). Or, replace the fuse block with a new one.
Have you added any extra elecrical load to the bike, like brighter lighting? Extra loads, draw more current through the main fuse. And, the closer you draw current to the fuse rating the hotter it will get.
When all is well, you should be able hold your thumb on the fuse and clips without discomfort. If not, you have some kind of contact resistance problem.
