Shower Knob Issues - How to resolve

The middle knob in the line of 3 to control the water doesn’t fit properly. It keeps falling off as the screw attachment isn’t long enough to tighten on. Can a replacement knob be posted to me asap please. Please see pic below 

Please see below – the level needs to be remove, when originally attaching it seems the internal grub screw has been tightened too much so that it will not allow level to attach correctly.  Remove the Lever and remove the screw from within the lever.  Then fit the circular knob onto the shower valve and using an Allen Key tighten the screw onto the shower.  Once tight then re-attach the Lever onto the fitted screw to complete installation