Boolean Union problem

 From:  MFort
Hello Michael
First thank you for a top of the line program and allowing me to participate in the beta. I'm just a recreational user, sort of gave up plastic modeling for the 3D world. Your program is very intuitive for the newb and I'm having a blast with it. You definitely have a customer here.
I'm having a weird problem though. In the attached model I'm doodling out an Armored Personnel Carrier. Inside where the tread will be I have eight road wheels. Two very large and six smaller. Inside the smaller wheels are cylinders I am attempting to Boolean union to the wheel structure. For whatever reason the cylinder will not union with the wheel structure. The tool finishes like it has done the union, but moving the wheel reveals they are not attached. I have already performed a Boolean subtraction to get the indent, does that make a difference? I'm attaching my save file.

Thanks Again Michael

and Thanks Frenchy Pilou without whom I wouldn't have found this gem of a program