My suppressed 8.5 has suddenly gone from eating anything I feed it to only wanting to cycle subs.  I run a very heavy buffer and it has never been a problem even when I ran without the can.  Do I need to run a lighter buffer when shooting supers or is there something else that may be happening to cause this?

More specifics will help with troubleshooting, but the biggest thing to look into is: what changed between when it WAS working and when it WASN'T working?

When something works, then stops working, something had to change.

Here are some things to think about:

Is it dirty and needs a thorough cleaning?
Is it lubed properly?
Did you change lubricants?
Did you swap a part? ANY parts at all?
Did you adjust any parts?
Are you using different magazines than what worked before?
Did you use different supersonic ammo than what worked before?
Did you check for parts wear?
Is the recoil spring getting weak?
Are the mag springs getting weak?

It could also be something like: it was always just barely on the edge of working/not working with supers, and getting dirty or a little mechanical wear slowed the action enough to push it over the edge into "unreliable" territory.

Out of curiosity, how heavy is your buffer?  Total reciprocating mass?  I mostly now use 7.5 buffers with a total mass of around 22-24 oz.  This seems to work well for my 4.5" and 5.5" builds,  both suppressed and unsuppressed.  I've gone as heavy as 25oz. total mass without issues..

^^^All good points^^^

Some more specifics would be nice.  When you say not cycling does it fire the round and not eject the casing at all??  Is it ejecting the casing but not picking up a new round?  Any stove piping?  Double feeding?  Something a bit more than just not cycling will help everyone pinpoint where to focus taking a look at when suggesting troubleshooting tips.

I'd back off the buffer weight and see what happens.  Assuming the gas port is the correct size for your barrel and we are working with a pistol length gas tube it should cycle subs/supers/ suppressed/unsuppressed with a carbine weight buffer.  Make sure your gas block isnt loose and its in the proper position as well.

I was way off, it was the trigger not resetting.  I remembered reading about the larue mbt2 not liking pcc bolts, but mine ran for a long time without issue.  

Swapped a spikes battle trigger and it chewed through 120 rounds of supers perfectly.  Lesson learned.

Glad you got it working again.