I'll second Lou's suggestion of the AUX 1 key.
Are you getting any noticeable fan noise? These dual stack units are a pain to fill. They will take way more than 6 drops just like anything else.
It is also imperative to have a needle tip filler, otherwise some of the fluid could leak past the funnel. The funnel is very large the fluid can get trapped in the base with an air bubble.
I'd put at least 10 drops and blow in one stack while holding your finger over the other. One end of the funnel is shorter the the other. That one will get the fluid to the unit a bit faster but you'll have to remove the shell to figure out which stack has the shorter end of the funnel.
Heres a pic from Lionel of the funnel upside down.
Its also #81 on the breakdown here. You can see how one end is shorter.