Hi drongo,
to GameBoomers.
There is a patch which addresses this problem, but if you purchased this game as new fairly recently then it should not be necessary. Check here:
Patch Otherwise, it's all about finessing the timing of this part -- it's tricky!
Did Carol identify Leonard? She needs to do that. Then run up behind Leonard and get as close to him as you can before whacking him on the head with the skateboard. There might be more than one hotspot for this.
and let us know if you still have trouble.