The Spike protein had evaded the apparently sufficient library of antibodies that were supposed to neutralize it. Thus, it is possible that some people do not make specific neutralizing antibodies after injection, and thus, the Spike protein is able to circulate and damage the body, specifically the heart muscle.
https://www.trialsitenews.com/a/circulating-spike-protein-that-evades-antibodies-f9cd66d1