Have you tried to do a trial assembly of the bearing, dust shield and circlip in the housing without the shaft. The circlip groove is wider than the circlip itself so if you have measured the thickness of the bearing and dust shield to the inside lip of the circlip groove you will find their is a little extra space when the circlip is pushed to the outside edge of the groove.
Also make sure their is no rubber lip from the bearing seal interfering with mounting it and shield properly.
Are you using a name brand bearing of the correct width? Has the dust shield come from a reputable supplier and is the correct thickness? Mistakes happen all the time by suppliers who don't understand the critical nature of some of the dimensions
In the end the housing may just have been machined wrong and need correcting. Do you know if it has ever had a dust shield fitted. The earlier issue 16 housings did not have a dust shield and maybe there was a transition issue 18 housing that was not machined for a dust shield either? That would be a very Lotus thing to do
cheers
Rohan