A couple possibilities come to mind: might have been too large of a pitch to begin with, what was your target gravity for the 1g? An 11.5g sachet is a standard pitching rate for a 5g batch of average gravity.
Second item: did you lower the fermenting vessel's temperature at any time to speed up the process of yeast flocculation and sedimentation? In other words, did you see a thin or thick layer of what was obviously yeast on the bottom of your fermenter? If not, it really just needed more time to finish settling out.
Third, Did you have a stable final gravity near the expected final gravity? Did you check it a couple times?
Fourth, and you can find this on this forum multiple times: yeast work and complete their job in their own time frames. they do not know that it is the 10th day after pitching, etc. Give them the time to get the job done ,and verify that the fermentation is complete before bottling
Welcome to the forum and the hobby
edit to add a big plus 1 to all of Pete's questions