You have a plan, you execute but the results (such as sales) are not there. What’s the problem? Bad salesguy, so fire him and hire a new one instead? Or could the problem be a more fundamental one: is your plan wrong, or more precisely, the assumptions your plan is based on are wrong?
Result = plan x execution. It’s not always obvious which one is wrong. Both could be. But for startups struggling with sales, the biggest issue is typically not about sales execution (it might be suboptimal, but not the root cause). It is likely to be a product/market fit issue. Or even more fundamental, problem/solution fit issue. If there is no real opportunity, not even the best salesguy can get a deal.
In most cases, rather than rotate through a number of salespersons, you should go back to your basic assumptions about the market, customers, their needs and expected behaviour. You probably have missed something. Learn from your experiments, adjust and then try again.