I had a problem with one of my Copics today. Unfortunately with one of my favorite ones (G21), and I haven't seen anything like it mentioned on here. I ran out of ink, and refilled. I have refilled before on other ones, just not this one. After I refilled, the marker seemed a little darker than it had been, and not as smooth as it had been. Today when I went to use it, it wasn't coloring much at all but was sticky on the brush tip, and coloring very gloppy. In fact when I colored, some was almost as dark as the G28 color, yet this was G21, and 3 hours later those dark sections are sticky. Yet the tip of the marker looks uneven, like ink isn't getting to areas. The other end looks fine. The tip is also sticky, almost like the ink is dried or too thick to flow.
I guess it almost looks like it I have picked up some type of glopped up coating on the brush tip. Are there any papers or inks that could cause this type of coating to be on the tip to ruin it so you can't color through it? I want to avoid this in the future if possible.