YES! This is exactly my experience. Even if I do stuff myself it is often very hard to get to consistency, there are almost always more variables in play than the ones that I'm initially aware of. I've seen seasoned pros driven to despair by things that 'worked in the lab' but that they could not replicate outside of it for want of a simple oversight. Sometimes the line between success and failure is hair-thin, you can be almost there and never realize it.