I want to use some stop squeak screws that pull the subfloor tight to the joists, then break off beneath the surface of the floor. I am having a difficult time locating the joists: my stud finder doesn't seem to work very well and I am losing my marbles drilling holes through carpeting and missing the joist. I am doing this on both carpet and vinyl.
The joists should be at the squeak, you just need to determine the direction of the joists. Typically, this is the shorter distance between exterior walls, unless there are interior load bearing walls used to break up the span. Some stud finders have a deep penetrating settings that might find a joist through carpet. And you can also try finding the joists in the ceiling below and measuring the distance from a common wall.
For vinyl floor, get a pair of very high strength magnets. Take one, slide it around on the floor until it appears to catch on something. Then, following where you think the joist may run (as posted above, shortest run between exterior walls), slide the other magnet until it catches something again. If you are successful, you will have a line between the 2 magnets perpendicular to the exterior wall. Mark this line, and try again on 16 inches either side of the line.
If this is the first floor of a house, then you can see the subfloor from underneath in the basement. You can guesstimate where the joist is, shoot a screw on either side, go underneath and measure where the joist is between the screws, go back up and use the measurement to hit it.
Those kind of screws were originally designed to re-attach a single board in a place where it had popped loose on a floor made of nailed-down individual boards (especially squeaky stair treads). I doubt very much they are going to do anything at all if you have a plywood subfloor. You would be asking a couple of screws to handle a load that 50 or more nails or screws are currently failing to do. Floors like this creak because they aren't sufficiently structured, and over time the nails loosen and let the joists rub against the subfloor as it bounces.
Use a long small (1/8" or smaller) drill bit and go under the house, drill up through the floor as close as you can get to the floor beam. Go in the house and locate the drill bit, now you know where the floor beam is and the very small hole is negligible. I also use this method in my sheet rock ceiling to locate the ceiling walls in the attic.
I believe joists normally run across the shortest width of the house - this should help you locate the direction correctly. Hope this helps.
Your stud finder won't work because you have 3/4" -- if not more -- plywood.
Q: What size screws are you using?
If your screws aren't getting embedded into the floor then you're not grabbing the joist. You need 3" galvanized or Deck Mate screws. Even drywall screws will work but the threads have to be coarse.
Galvanized screws or deck mate screws are what you need, 3" screws are what you need even Sheetrock screws will work but the threads have to be corse.