Can you take the switch out of the equation? Usually when sniffing, you'd want to use just a hub since it's "dumb" (not so smart that it'll try to filter packets that don't need to go somewhere, e.g. the Windows PC running Wireshark). BTW, our support team has lots of experience helping people sniff packets with Wireshark.
But I'm also curious about what the bigger problem is you're trying to solve. Can you share a bit more about your application? I'm guessing something isn't working quite as you'd like, hence the need to see/sniff what's happening to troubleshoot? I hope you'll share!