To confront all such occurrences of mutations, one should always pick up at least 5-7 clones, can even go up to 10 to get the right sequence of interest.
If none of them are right, please consider to change the cloning strain. This situation is highly empirical and sequence dependent. One cannot compare to past experiences. Each sequence is different in its foreignness to bacteria.

BL21(DE3) should not be used for cloning. This is an expression strain that is lysogenized with phage DNA and may not allow foreign DNA to be cloned correctly. However, it is good to drive expression in this, should you already have the right clone. Other cloning strains to try are DH10B, TOP or stbl2 cell line. Alternatively, you may also try to incubate at a lower temperature (25-30 degrees C after transformation) to slow the growth of the bacteria and increase the chance of cloning a potentially toxic insert.