Do you mean it's easy with your road bike on a trainer? In that case, I think it has to be position. Take plumb bob and measuring tools with you and set up the spin bike just like your road bike.
If you mean it's easy with your road bike on the road, I'm the same way. I don't climb well until I've been on my bike for an hour. It takes that long to get my juices flowing. On my rollers with resistance, I warm up for a 1/2 hour in zone 2, including 1.5' intervals in the gear I'll use for my real intervals at 20 and 25 minutes. After that warmup, I can usually go anaerobic after 5 minutes of high cadence hard effort. And after that effort, it gets easier. If my legs aren't up to that first 5 minutes of hard effort, I bag it and try again the next day.