I just bought a new tip (Weller ST7) for my soldering iron. I've tried tinning the tip, but the solder just keeps forming a ball and then falling off onto the workbench. What am I doing wrong? (I never had any trouble with the old tip.)
While having a temperature controlled soldering iron is always better, even a standard station like the Weller WLC-100 is more than adequate to perform soldering. There are some excellent suggestions in the comments but I wold also add that you really should use some additional flux. You can get No-Clean flux for general purpose usage or some RA/RMA flux if you work on corroded/exposed wires (i.e. point to point wires, guitar pickups, old electronics etc.)
Most good quality solder has flux inside it so you should double check your solder reel or the website of the manufacturer. If it doesn't have flux, you should get proper solder from Multicore, AIM, Kester or MG Chemicals. Avoid the cheap imitations available on some websites.
So use a tip cleaner as Steve G suggested, clean the tip regularly with a brass wire cleaner like @Bimpelrekkie suggests. Don't be afraid to add flux and make sure you have good quality solder wire.