Well, my N-400 application, which was breezing along (PD in mid-September 2005 and interview scheduled for early May 2006) hit a wall when the USCIS descheduled it.
In the end, it turned out that the USCIS hadn't delivered my file to the Dallas DO in time and so they descheduled my interview. However, it took 4 infopasses and the intervention of the offices of my congressman and Sen Kay Bailey Hutchison to find out what was going on and to get it back on track. Had the Senator's office not intervened, I'd probably still have an expired GC in my pocket. Luckily, with that intervention, I was able to get an interview and took they oath right before Thanksgiving last year. So, I'm not expert on what to do - only on what can go wrong.
My experience with my daughter's N-600 has been equally disheartening. My wife's interview (early May) and oath (June 1) happened on schedule and my then 17-year-old daughter got automagic citizenship. We expedited a passport for her in June. Then we submitted an N-600. That was the end of June. Ouside of a frustrating InfoPass last month (see the last thread I started) we've heard nothing. The Dallas DO claims to be processing August or September N-600s - but ours is in limbo. Who knows why? N-600s would seem to be particularly easy to process - no FPs, no name check; the officer only has to inspect a handful of documents (birth certificate, marriage certificate, green card and something that proves parent and child live together (in our case, matching TX drivers licences)) and say "oh, ok".
Oh well. The only thing you can really do is sit back and look in your (USPS) mail box each day. Unless you are *very* far behind schedule, it's hard to enlist someone to your cause. If we don't hear anything about my daughter's N-600, I'll probably start writing letters soon. I *really* want this to get settled before she heads off to University in the late summer.
I've been anxiously looking in my mail box *every* day since mid-September, 2005. It's getting a little old.