It's pretty simple in modern times with modern surveillance. A well connected Militia is a house of cards ripe for falling down. Only by shrouding local militia organization and training in secrecy will the rest of the militia be safe from the infiltration. Take an organization like the Michigan Militia. They have a rank structure, a lot of members, and they are well known. It's probably also safe to say the feds keep tabs on all of them. 3-6 people per local militia, each of whom has known the rest for years and trusts them, that is the largest I think an organized militia should be these days.
I know that might come off as paranoid, or downright idiotic, but given who is in DC now, organized militia activities need to be kept secret.
In the hopefully never to occur scenario where the militia has to "go active" the only way any success could be had is for actions to be kept independent and separate. Uniting would be the wrong approach because depending on the scenario it could either mean mass arrests or mass killing of the militia men. The only thing that would unite anyone would be the much needed push to spread information around about who the enemy was and why.