They may be loose because the screws lifted the cabinet slightly.
Fixing bathroom sink to wall.
The problem is the wall is plasterboad and the fixings supplied are for a solid wall.
If so replace them with slightly thicker ones that you have to tap in with a hammer.
Amazon tool shop here.
If the wall was tiled replace the tiles and grout.
I have a sink basin for a bathroom to fit.
As with any heavy object attached to a wall over time a sink can become loosened from its anchored state on a bathroom wall or under a kitchen cabinet for example.
Drill the holes with a 5mm wood bit.
Fill the sink bowls then as they drain check all joints from the sink to the wall with a dry tissue see photo above.
Drive the screws into the new holes and sink them into the wall studs as far as they ll go without over tightening them.
Offer the sink up to the line and get someone to mark the screw holes through the sink which can be found at the bottom.
Squirt white silicone onto the back of the sink and place onto wall.
The wall is plasterboard and the sink is fairly heavy it also is the freestanding type ie the pedestal is not to the floor.
Insert wurly fixings into the holes these are known as metal plasterboard fixings at the builders merchants.
Now that you have made the repair to the water damaged bathroom wall all that remains is to finish off.
Wall mounted sinks have a great modern look and make a small bathroom especially a guest bathroom seem less cluttered and more spacious fortunately mounting a wall sink is a doable project for most diyers.
Many people may try to band aid the problem by recaulking the edges of the sink backing to the wall.
Finally cut and install new base trim before applying silicone to the inside edge where the drywall and tub meet to create a watertight seal.
Run and drain the dishwasher and check the waste hose connection.