The armed suspect who was arrested on gun charges outside Donald Trump's Coachella rally in California was happy to talk to the sheriff's deputies who busted him, but he wasn't so chatty with federal agents ... because they made him wait too long for an interview ... and now the feds don't have a motive.

Riverside County Sheriff Chad Bianco tells TMZ ... the suspect, 49-year-old Vem Miller, had a cordial interaction Saturday with the deputies who interviewed him, and he was cooperative.

The Sheriff says deputies specifically talked to Miller about the guns he allegedly possessed, because the weapons were the reason he was arrested ... but protocol calls for federal agents to figure out the motive.

Problem is ... Sheriff Bianco tells us the feds took a long time to show up and talk to Miller, who was growing more frustrated and angry the longer he waited. Relatable.

By the time the FBI and Secret Service arrived, Sheriff Bianco says Miller refused to speak with the feds ... so agents weren't able to extract a possible motive from the guy. D'oh!

Seems the feds were dealing with a man with very little patience for bureaucracy ... even after two thwarted assassination attempts this year.