I'm a little concerned about some of the replies to this question.
Even if you re-test retained samples at the laboratory, there is a chance you will get negatives. As Charles C. mentioned above, it could be several factors, but the simplest one is that not only is Salmonella not evenly distributed, but it is at a very low level. You need a single viable cell for a positive test, so if there were one or two, it will not be distributed in the other samples, and it will not necessarily be in the retains either.
A quick blurb about the test: generally your sample is weighed out to the desired weight of the test and then enriched with a broth. The samples are incubated overnight at ideal temperatures. So if you have a viable cell, it will be happy and reproduce. The sample will have a much greater number of bacteria the next day as a result, and will be more easily detected by laboratory methods used. It only takes a single cell. Re-testing at another laboratory and getting negative results will give you a false sense of security and you will release product that is not safe.