Cerevo has officially confirmed some switchers are compatible with FlexTally.
See this page to find the list of tested switchers.
However you may not connect with a switcher on the list when your firmware is not up-to-date. Please check if your firmware is up-to-date when you cannot connect with your switcher. If not, update your firmware and try again to connect with the switcher.
See this page to find steps to update firmware.