Sagging Floor Repair

Hello,

My 900 sqft bungalow's main floor is sagging to the middle and vey squeaky. The lowest point in the middle is an inch lower than the side/ wall side. The basement is finished with drop ceiling. Luckily, i was able to remove some ceiling tile and checked in a small area of the floor joist. Seems there is no water damage or any other issue; such as cut through.. Please see the attached photos that some joist has scrap/filling but joist next to it has a gap that was not filled. Could this be the cause of floor sagging? There might be more joist like this. What is the best way to fix it securely?

Thank you in advance your valuable advice.