It's not uncommon for Congress to not vigorously pursue defiance. Often the subpoena allows them to work out other things that can be done to facilitate oversight. In the case of trump's men, though, they are pursuing only those couple that are both likely to finish soon enough to be useful (a decision in twelve months isn't helpful) and most likely to go their way (withdrawing when they get unfavorable judges).
It sucks because I want a more aggressive strategy too but this does seem to be more prudent.
Also the defiance is plain obstruction, they don't need to force testimony for that.