The top of my Innokin Adept Slide no longer opens.

It's the oddest thing. You know when you take the top mouthpiece off and there are a couple red arrows? Usually, when you push by the red arrows, the top opens and you can fill the device. But all of a sudden, no matter how hard, I push, the top piece doesn't slide to open. Has anyone else experienced this or does anyone know what might be going on? It's a riddle wrapped in a mystery inside an enigma.

Original source: