Opportunity solution trees, interview technique, assumption testing, and avoiding confirmation bias.
3 questions in Product Management.
You look for evidence of the behaviour, not enthusiasm for your idea: interviews about past specific episodes rather than hypothetical futures, triangulated against what usage data, support tickets and lost deals show people already do.
Treat blocked access as a research design problem rather than a blocker. Make the account team's job easier so they open the door, triangulate the proxies you can reach against behavioural evidence, and state explicitly which decisions your sample cannot support.
Establish whether the test could ever have detected the effect you cared about, because an underpowered null tells you nothing; then read the confidence interval rather than the verdict and decide whether to extend, test a larger change, or accept that the assumption was wrong.