Probably you have it set to "BG Only," which is the stabilize mode (wish they would have just called it that). Try piping your imagery into the Foreground input, then setting it to "FG Only." But don't disconnect the Background—the Tracker requires that to set the reference size, and it will fail if nothing is hooked up there, even if the pixels are being discarded.
Details about the Operation page of the Tracker begin on page 663 of the Fusion 9 Tool Reference doc:
https://documents.blackmagicdesign.com/ ... erence.pdfIt will look a little different in Resolve—the multi-buttons are all combo boxes there, for instance—but as far as I know, little to nothing of the actual operation of the node has changed.