Have you looked at the final contig and see which part is missing? Does any of your other four sequences contain a nearly similar sequence (e.g. one bp difference) than the missing part. Velvet uses k-mers (provided by the option 'hash_length' with velveth), if you have a k-mer that is near identical, the assembler does not know which path to choose. Example;
GATAGAGTAGAGA
GATAGAGTAGAGT
The assembler does not know if it should choose the A or T here. Look at the last x bases (where x depends on the set k-mer) of your contig, and see if any of the other four reads contains this k-mer.
Regards,
Boetsie
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