Spinning Wheels - Model Railroader Magazine

Hi Guys. I have a question about several N gauge locomotives I have. These are all second hand, used enginesbut seem to run well except that the wheels spin with a small load( a few cars). They seem to be heavy enough, and on a few there is so much lead there's not room for more. Do I need wheel tires or frog snot or what? These are different makes such as Atlas,Con-cor, Lima. Any help would be appreciated, thanks.

Jack