Some casters may be a softer material that takes more effort to move them. IF you welded them on did you connect the ground to the frame or caster, you may have arc'd across the swivel bearing.
This thread warmed my heart. Let me add a little warmth as well. Here is my Ellie Mae (tatertot) the mastiff and Stella the pig. Don't let em fool you they're a team Ellie trips em and Stella attacks.
Andy