Humidifiers and Dehumidifiers - Generalaire E2 humidistat not working...not enough power?

I have a Generalaire E2 and a Generalaire bypass humidifier. When I hook the E2 up directly to the furnace/humidifier/xformer via about 4 feet of wire, it works perfectly. When I hook it up upstairs on about 25-30 feet of wire it does not.

Upstairs, it powers up as normal and reads the outdoor sensor fine. It tells me in/out temp and indoor humidity. When I set it to manual mode and adjust the humidity level up (even all the way up to 60%), it does not throw the little relay to turn on the humidifier.

I have replaced the wire and I know the wire is good.

I measure 25-27 VAC at the main floor location, and the same downstairs on the 4 foot piece of wire.

I'm thinking that I have enough voltage but not enough current available at the end of the wire run to turn it on? It's a 24VAC 12 va transformer.

Tomorrow I will go get a 'jucier' transformer and try it maybe 20 or 25 va - but 25-30 feet is not a very long distance and it seems strange that it won't work when it works fine on 4 feet of wire.

Any suggestions?

What gauge wire you use?

Yeah, you may need a stronger transfromer.

further snooping has shown that it will not operate with the outdoor sensor attached, even in manual mode, which doesn't use it. Have tried a replacement sensor.