WebGL problem (I think)

I have a web game with a drag and drop menu, the menu buttons are pinned to an invisible sprite with drag and drop behavior. It works fine when I test it on different computers and browers, but some users have reported that the menu items are invisible.

When I experimented, I tried turning WebGL to off instead of on, and then the menu was invisible for me too. So I guess those users computers can't use WebGL or something like that?

I discovered that the menu buttons are not invisible if they are on the top of the Z order in the layer, but the thing is that I have a mask on either side of the menu (invisible sprites with "source in" effect, and I need those to be on top in the Z order for it to mask the menu properly.

Any advice on how I can make it work for those without WebGL support?

Thanks!

When I read about WebGL here https://www.scirra.com/manual/146/effects it says that only effects will stop working when WebGL is off, and blend modes should work anyway. So is this a bug maybe? Because I don't have any effects, only the blendning mode on the sprites that is on top of the menu buttons.