Those mason jars like many other jars have a concave on the bottom that will throw the mixing bar off the magnets if spun too fast. The kind of bar your using also hinders its performance, the bar with a rib in the middle gets thrown all the time and I rarely use it other than in a beaker. I found the totally smooth ones work best in the jars I use at a slower speed, you also have to play with positioning the jar to get the bar to spin nicely. I make 100mls at a time with a jar and made 250mls but with a beaker and warming up the VG first. Cutting off the fan blades by scoring each side and snapping them off increases spinning speed since there is no resistance from the fan blades. You could have a lower RPM fan I went looking for fans and found there are 1800, 2000 and 2500 RPM computer fans so maybe you have a lower RPM fan that doesn't have enough knards to spin fast enough. I like your 420ml size batch but IMO I don't think even with heating up the VG a computer fan magnetic mixer will mix it properly especially once the VG cooled off and thickened up. Put a flashlight to it once started mixing and you can see how much of the mix is mixing and if the top part is mixing or not.
