Hello,
1)the output only show main effect and its level that causes fails, but no 2-factor-interaction. Why with t2 in this case , only the main effects reported?
The analysis works on the principle that the simplest explanation (involving the fewest number of factors) is where to investigate possible failure-inducing combinations. In this case, since there are main effects that would explain it, this is where it would start. Empirically it makes sense to investigate those first, but in the future we plan to add the ability to increase the combinations looked at.
2) there are couple of known combination that will cause fail were design into the test, but the analysis didn't show those factors and its combination that will cause fail. Is this an issue on Covering array?
The big issue here is that there are so many rows with failures, it still thinks there are single factors that could be the cause. In future versions we are planning to add the ability to examine a higher order of combinations. In this case though, there are so many failures and so few successes, even if you did this the list of potential causes would be nearly all two-factor combinations, because very few combinations can be removed from consideration.
The key is to have some more runs with successes to reduce that set of potential causes. You can get a better sense in Section 4 of this paper: https://rdcu.be/cv7tv
Hope this helps.
