Is it possible to identify which part(s) out of the 29 are problematic, by small step method by trying symmetry on half of the pieces and so on. It could guide the search...
Another absurd proposal, the assembly is fully solved (no more little blue pen in the Feature Manager) and the components in the local cache?
A screenshot of the Feature Manager of this assembly could also help us.
Personally I have already encountered this problem x times, the only way for me that I have found to solve it is to make a new config and combine all the other bodies before symmetrizing.
Then apply the "body-erase/delete" function. in order to erase the basic bodies of symmetry.
If you then want to add other functions such as holes or sheet metal, you have to raise the bar above the combiner function in the original part (original config). and then sober up the functions in the symmetrical config.