I'd dig up that area where you believe it to not be backfilled properly first and see if that is the cause.
I had a probel m not long ago in my basement as well, I starrted cutting out the floor to install a french drain system in the floor to a sump pit. I woudl have attached the cause but there is concrete galore aroudn my house, plus a carport on one side and a large patio on the other, it woudl have been in the thousands just to dig out and replace the concrete, plus hundreds of hours to take the carport and garage down to get a backhoe in there.
So I did the french drain method, I foudn a rusted pipe hoooked to the sewer line, except the sewer line went out the back of the house and this line was connected to it but ran towards the front.......
I cut the pipe near where the sewer goes out, filled it with concrete, the other end where it went out the frontdrains right into one of my trenches and eventually makes its way to the sump pump.
Got a big rain the day after I finished the entire thign (2 1/2 yards of concrete) and it worked perfectly. A rain just liek we had that day would've coated the floor of the basment with water maybe 1" deep.
There is actually more to that story right now but I can't comment for legal reasons.
In all I had 70' of trenches dug 16" wie and 18" deep.
The digging wasn't bad, the cleanup sucked.
To do it, go get a gas powered concrete saw with water attachment and a few blowers nd some good ear plugs. Set up blowers to evacuate fumes from saw.
Dig trenches 16"-18" deep, line it with landscape fabric (permacloth), put in 2"-3" of rock, put in perforated 4" pipe with eh holes pointed down, fill with rock stopping just shy of where grade will be, wrap cloth over the top, add another 1-2" of rock, put down plastic sheeting, install wire mesh and spike rebar into existing conrete then pour concrete.
The rost part is hauling out the dirt you dig out and hauling the rock in. The easiest part is putting the piping in.
Tip #1, keep the place clean as you work, it makes the final cleanup easier if you have ket it fairly clean as you go along.
To all of the above, don't ask me how I know!