pushes the switch rail to the other side allowing it to exit.
It seems tolerances are at play. The pony axle back to back wheel spacing may be a bit too wide (nominal back to back spacing is 1.575 inch) whilst the spacing of the switch point rail to stock rail may be somewhat tight.
I can’t see how the back-to-back spacing, or the point rail to stock rail gap, affects whether the pony wheels push the switch out of the way, or not in this case.
Tim, if you google “site:largescalecentral.com spring switch” you will find a few threads where folk tried to make a spring switch - which is what you have here. In my experience, sometimes they work and sometimes they don’t. Sometimes a loco works, and sometime it doesn’t. (Sound familiar?)
Not only is the weight of the pony truck important, but also the spring of the switch. There may be dirt in it. The ‘spring’ (whatever it is) is now a year older and may have been stored so it isn’t working the same as it did last year. In your case, the spring has to be just enough to return the points after the train passes through - no more, no less. Once you have established that springiness, then you can add weights to the truck until it pushes the points out of the way.
And you can expect to have to research this every year (http://www.largescalecentral.com/externals/tinymce/plugins/emoticons/img/smiley-cool.gif)