Sometimes increasing the current and weld time seems to hit a wall as far as nugget growth goes. It is very difficult to grow a nugget larger than the face size of the electrode.
A method that some find successful to increase the face size is to use weld impulses. At the end of the weld cycle schedule an off period of one or two cycles then repeat the weld schedule. The length of the impulses may need to be adjusted to prevent overheated and expulsion.
A larger electrode face size is also an option. This will require a completely new weld schedule. The larger weld face will encourage a larger weld nugget.
Reference: RWMA – Resistance Welding Manual 4th Edition