Models showing up

[Elizabeth S]

I am importing a model of a car and each of its wheels separately into my script. I have created animation paths for both the car body and its wheels. When I run the script, some of the wheels may or may not appear in the scene. Running the script another time (without changing it at all) yields different results - either it may have randomly worked this time or a different wheel(s) may not appear. Any ideas what could be wrong? I'd greatly appreciate any suggestions to clear up this frustrating problem. Thanks.