I have been using the chromosome browsers on several websites to try to find hints to family connections, plotting the results on an on-line tool called DNA Painter. I have an instance where the autosomal DNA results from two individuals clearly overlap, but the host website doesn't flag them as a match. What information is the website using to reject the match, and if it could be useful, why can't it be made available?
Determining what is a "match" is not necessarily that easy as there can be false positives and false negatives. All the companies set a minimum threshold for declaring a match so in theory there will be some false negatives that don't meet the companies' thresholds. However, there is a bigger problem with false positives. This is especially the case when the "match" is only on a small segment, and particularly on segments that are less than 15 cM.
There are now additional problems because of the different SNP microarrays used by the testing companies which have a low overlap. Companies have special techniques such as phasing and imputation to try and overcome the low overlap but there is inevitably a high false positive rate on the smaller matches.