I recently came across a great quote on marketing by Seth Godin, but it is equally true for grappling training:

“Persistence isn’t using the same tactics over and over. That’s just annoying. Persistence is having the same goal over and over.”

So if your goal is to armbar your training partner but he’s always countering you, don’t change the goal – find different setups and techniques to achieve the same goal. Your armbar technique will thank you for it.