wireless need more details to be understood. Low signal can mean a range of things. Interference, distance to AP, bad clients, building properties (like metal grids in the wall), amount of clients in the SSID, the amount of SSIDs, channel utilization (2.4GHz vs 5GHz) etc.
one good guideline is to have as least signals on the channel as possible. As such, having 5 SSIDs on the same AP or on the same channel with upper and lower floors IS going to interfere and affect client experience. That is not a FortiAP thing, but general wireless design.
Making less SSIDs and having clients connect and authenticate, being separated by VLANs is the best case, but not always possible. 2-3 SSIDs are practical, the less the better.
5GHz is better but has lower range. Better because it is usually less used with clients and hotspots.
Use a wifi analyzer to get a rough overview of channel usage:
Better is normally to get a proper site survey (these are not free service) prior installation of the APs. If you have only that AP, I would place my bet on the channel interference, the app there should show you already.