If you aren't able to increase the audience size, which it doesn't sound like you can, you may wish to consider the difference between the A/B variants. Are they dramatic enough to draw different results? Sometimes people use tests that are too similar so its hard to get different results. I would suggest using stronger variations when you have a smaller sample size, such as: Plain Text looking vs HTML Subject lines that focus on different topics - If you have an email that has a single topic, find 2 different angles to presenting it (Such as (a) Save money by doing these 3 things and (b)Read this case study to understand how improve your processes)
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