Welts depend on a lot of factors. First up to actually welt, that is get raised lines left after caning you must obviously be using a cane. Anything broader than that like a paddle has too wide an impact and does not cause the same effect. It is sometimes possible with a crop but they require a lot more hit power to get a welt to rise.
Secondly there is a big differences in the individual, really fleshy asses don't seem to welt as easily as less fleshy ones in my experience.
And finally of course, technique. A good hit will strike between 3 and 6 inches of flesh simultaneously so the cane needs to strike so it curves with the flesh.
Like most things practice makes perfect!