In a low wing set-up, the fuel gets "sucked" up by the fuel pump(s). Hence no "both" setting - try to suck up from an empty glass and a full glass simultaneously through a pair of straws, and you will only get air.
In a gravity fed system, there is a positive pressure in the fuel lines. That pressure does not really change that much, it comes mostly from the fuel in the line (a couple of feet above the engine) and not from the fuel in the tanks (a few inches in addition to that).
In general, it helps to put the fuel selector to a single tank
- When refuelling. With it on "BOTH" the tanks are linked, and a small amount of fuel will flow across.
- When parked on anything than level ground. Otherwise the lower wing tank will be VERY full, and if it is the side with the vent [if there is only one] you can lose a lot of fuel overnight
On some aircraft, the OFF position does that as well, on others it does not shut off the link between the tanks, so use whatever the POH says.
but i will say some time ago i flew a 172S which for some odd reason wasn't taking fuel from both tanks evenly (it had the G1000 suite so i had visual and audible warnings) it was giving a low fuel indication on left tank but 3/4 full on the other, instead of taking BOTH on the return leg take off i went for FULLEST didn't fancy a cough from the engine climbing out with 4 pax on board...
The most likely reason for this would be flying out of balance. Even a single degree of bank, barely noticeable on a murky day if you never look at the balance ball [or triangle in the G1000], can lead to significant fuel imbalance.
One degree of bank will make the outside end of the lower wing tank sit a couple of inches lower than the higher wing tank, and fuel will flow down... although empty on one and 3/4 full would be a bit extreme; 1/4 to 3/4 would not be unheard off for the less sensitive pilots. Especially when they fly uncoordinated steep turns or holds in an air exercise...