I have made a small test font that has two axis (weight/width) and three masters (regular, bold, extended). If I export the font from Glyphs, the axis sliders are shown in fontview but aren’t variating the outlines. I tested the font in Webkit nightly and it worked fine. If I add a forth master (bold extended), then it works in fontview, too.

BlaBlaGX.ttf.zip
Only the uppercase A has outlines with variation.