I know you can stack regular creeps by getting them from their camp before next minute, so that if they are not in camp for example between 9:59 – 10:00 new creepts will spawn there.
However this doesn't always work for ancient creeps. Sometimes they spawn, sometimes not. Why? Is the timing different there?
Best Answer
The timing isn't different at all. Could you be more specific as to which side you've tried to stack from, and failed? If you're on the Radiant side, it is far more effective to run straight south along the ridge instead of turning north towards the secret shop. If you turn north, you force them to take a path that results in them gaining less distance, and they go home faster. They will usually get stuck on the trees a bit while trying to chase you, which is what causes that.
If you are playing a character with a long range physical ability, like Beastmaster, then casting your spell from "behind" the camp (where the secret shop is, in the Radiant, or directly from the offlane in the Dire) and targeting the spot where the creeps are will also work 100% of the time if you time it correctly.
If you were in the middle of a real match and your ancients would not stack, and you're quite sure that you performed the stack properly, then someone probably warded your ancient camp. You will want to buy sentry wards and try to remove it. It's best to look up videos for good ward placement, as you can easily accidentally block your own camp while doing this. EDIT: @ywm's suggestion in the comments to the OP has some very useful graphics regarding this topic.