Pin option missing on fb personal page. Will pin if immediately select after post uploaded, then after short period of time, it becomes un-pinned. Why? Everyone I spoke with does NOT have this issue. Please help.

Please help. I no longer have the pin option on my fb when i pick the post i want pinned. The only way i can pin is if I immediately click the 3 dots on the top right after I post. It does pin itself but only for a short period of time, then it "un-pins itself. I tried multiple ways. I tried using chrome, my "chrome book" (laptop/tablet, my iphone, and the "desktop" mode. NOTHING works and I cant find a single solution anywhere I search and anyone I ask. Please help. Eveyone I asked and have talked to does not have this problem. Why do I?