My ex-husband receives supplemental security income. We have to children together. Child support agency says they can't get child support from him because he is on SSI. My question is why is this, when he gets about $700. A month lives with his mother, who also gets disability and smokes marijuana, his whole check goes for alcohol and drugs, in and out of jail. They give him money for that but not to help his children. Why?

Its not meant as a reguler type of income. Its for those that have no other way. It's a very minimal amount. Had he worked before all of this and for how long?

"They" don't give him money for drugs and alcohol. That's how he chooses to spend what "they" give him - and his family apparently is in agreement.

The Court won't attach his SSI? They will under certain conditions in NY. Where are you?

You can't get SSI or SSDI while actually in jail, so if he's in and out, it stops and starts.
Are you sure it's SSI and not SSDI? $700 seems a bit high for SSI.