The most common source of leaks is the valves. The rubber gaskets can deteriorate or you can get sand/grit on them so it does not seal properly. Try cleaning the rubber seal, inflate, and then put some soapy water on them to see if leaking.
Air temperature has a big effect on inflation too. In the daytime sun then can be nice and firm and then go soft in the cool morning.
This is very good advice! Per SR's advice, inflate the raft so it's rock hard. Then mix a bit of dish soap (a few drops) with water in a spray bottle. Spray direct into the valve openings---if you see bubbles, you have a valve leak----go to a marine supply store and purchase a bottle of 303 Aerospace Protectant--- spray the 303 into the valve opening(s), place a rag over the opening, then depress the valve stem several times to allow air to escape. Inflate the raft again, repeat with additional 303 Protectant, and depress the stem with a rag over the opening (so you don't get 303 in your eyes...)
Next, take your spray bottle of soap and water and spray both pontoons. Do this in small increments, so you're methodical. Cover every square centimeter of pontoon-----The seller who swore his raft had no leaks might not have been fiddling you afterall----he might not been aware of some pin-hole sized leaks (or seeping valves)
IF you find leaks, mark them with a pen. Once you've gone over the ENTIRE inflatable raft with the spray bottle and found your leaks, purchase a hypalon repair kit ( hypalon patches + hypalon glue).
Do not use glue specific to PVC---you want true hypalon glue (such as Bostik makes) that is specific to DuPont Hypalon. Mercury/Quicksilver make such a repair kit---eBay has them
Rule of thumb: cut patches into circles; allow at least 4 to 5 cm of patch material around (beyond ) the hole/tear/pin hole etc you're trying to patch
Mark the area to be sanded on the pontoon---sand just enough to remove the hypalon "sheen" on the pontoon and the sheen of the patch (no more then that---too much sanding = no good) Wipe off pontoon and patch with acetone, then follow directions on glue tube---generally, one thin coat, let dry, one more thin coat, then apply while still tacky and apply pressure to keep air bubbles from forming
I have piles of Avon patch material----if you need patches, PM me
good luck