The 31-year-old, who moved from Spain to Adelaide in 2013, was hoping to finalise his citizenship during the off-season.
The Reds were eager to assist in order to allow them to sign another foreign players, given A-League rules only permitting clubs to have five visa spots, but have hit a mysterious bureaucratic snag.
"I passed the citienzship test a couple of motnhs ago and I'm still waiting for the confimation letter to set up the ceremony," Isaias said ahead of United's FFA Cup semi-final against Bentleigh Greens on Friday.
"Nothing has happened yet so I'm not sure what's happening. I'd like to get it as soon as possible."
He added: "We tried to send a few emails. We tried to call a few people in the government to see if something was wrong but we didn't receive an answer yet. Hopefully we have it soon."
United have filled all five spots this season with Isaias, Ken Ilso, Michael Jakobsen, Mirko Boland and Baba Diawara.
"It's important for everyone," Isaias said. "For me it's important for my family but also the club to have one more spot for foreign players."