So there seems to be something up with my background check that causes longer than normal processing for NFA items. This also pops up from time to time with normal firearm purchases, I often get delayed with handgun pruchases. I have renewed my CCL so the firearms purchases are no longer an issue since with a CCL here there is no NICS check. My last From 4's were both approved in January but were submitted in March 2020.

I emailed the nfa division and got a canned answer on my current Form 1 that has been pending since 12.22.20. What's the easiest way to find out what is causing the extended background check delay? I have read that calling your Congressman helps but not sure if that is the long term fix.

Read the complaint thread. Delays are just standard issue these days.

Contact your congressman.

There is no long term fix as the FBI doesn't help with NICS checks for the ATF as they consider it a "courtesy" to them.

My guess is the best you could do is file for a UPIN (which the FBI website says they're processing ones from 2019 now - over 2 years slow...) , but I doubt that will fix anything as you would have to know WHAT is delaying you first.

Trying to find out what is delaying you is another problem in and of itself. You could request an identity history summary from the FBI, but that will take time as well, and there's no guarantee you will get exactly what is delaying you. Even if you do, YOU then need to get the information cleared up, and file a VAF for a UPIN... which again takes 2 years at least..

So again, contact your congressman. From my past experience they magically can get things moving. Mine contacted the head of the FBI NICS division and head of ATF NFA division on my behalf, and he forwarded me emails from that same exact day from both of them saying my background check was approved, and my form 1 was approved.

Probably something in your identify history (including incorrect/wrong info) that is pegging a metric. You can request it from the FBI. https://www.fbi.gov/services/cjis/identity-history-summary-checks

I just got an email from an examiner that the FBI is at 9/22 now.  I’m middle of October for my form so hopefully not too much longer but I’ll bet it’s more than a month.  They said my form 4 from July is waiting on the same FBI check since it didn’t make it over to them until Oct as well.

Contact your Senators, assuming you are not in one of the Demoncrat communist states.
They can do a FBI inquiry on your behalf.  I have done this on 4 separate suppressors and the interesting thing is once the FBI gets the inquiry, magically the background check happens that same day.  

Do you hold a security clearance?  No need to answer here but I had a friend who had one and it delayed every NICS check whenever he purchased a firearm as only certain folks were allowed to see his info.  

A clearance has nothing to do with it, the check is criminal in nature. I know people with a clearance that have never been delayed and that was for NFA and non-NFA items. If you buddy is getting delayed it is not due to the clearance.