Although it doesn't necessarily signal a problem, squeaky stairs can be very annoying. It's easy enough to ignore, but wouldn't you rather fix it for good? Luckily, the fix is fairly simple for any home DIY'er.
The simplest fix is to use wood shims (available at most hardware and home improvement stores) to fill the gap. Try to press the shim in by hand. Use a hammer to press it in if it is being difficult. The end of the shim may be difficult to hit with a hammer while it is up against he stair step. Place a small wooden block in front of it to make the process easier. Break off the excess material from the shim after it is in place.
Another method is to insert additional screws. Drill a pilot hole at the bottom of the riser into the step just below it. Place the screw and test the step again.
As a third option, you can mount a small block in the angle between the riser and tread. Mount it to stairs using a screw and some carpenter’s glue. Make sure to drill a pilot hole for the screw.