Does anyone else have this problem?  It seems long storage seems to affect zero.  Also every different brand of 55 gr ammo shoots to a different POI.  Any comments?

paspecops...yup mine all do the same thing too(g).  Sometimes I think they're just mad cause they haven't been taken out for some exercise.  If I remember to run a patch or two thru the barrel it sure seems to help tho'.

About the ammo unless it's mil-spec stuff it WILL all shoot to different zeroes.  You can't expect this stuff to be EASY now!  Get out there and find out where your stuff shoots.  Or settle on one brand and become a snob.  Gotta have a plausible excuse to become a firearm relationship counselor.

That is a mystery, I thought I was losing my mind when my scope didn't hold its zero after a few months of non-use.  Then again that was with a Tasco scope.

The biggest change in zero with the tasco was noticed when I came back home after a five year tour.  Had the pops keep the Rem 700VS .22-250 in the cabinet, he never touched the gun other than to wipe it down with a silicon cloth every now and then.  Took it out on a session and when I got back I had to really question my dad if he had ever dropped the gun, or anything that could have knocked it off zero.  He swore up and down he didn't. (I believe him...now!)

Switched over to a Burris last year, followed the instructions by not putting loc-tite on the ring threads.  Shot all summer long, but when it came to pull it out for yote season, the zero had changed.  Rings were loose! That is what prompted me, that day, to buy an AR-15.  I knew its zero wouldn't change!

Spending some more time with the Burris, I now have tightened the screws down as far as humanly possible, to the point where there is no light seen between the two ring halves.  Loc-Tited the things and haven't had a problem with zero change since.

I think zero change is due to two factors:

1. Quality of scope
2. RING TENSION, I think very slowly over periods of time, the screws unthread due to the up force  of the top ring half.  

Solution: Loc-Tite

NOTE: Burris claims one should not use loc-tite on the threads.  That is complete garbage.  While I was at Perry last year, I saw individual stands of loc-tite and spoke with many shooters who use it on their rings.  What is up with Burris?

You didn't say how much your zero is shifting and at what distance. Or if it's from the bench on sandbags / rest, or from a prone supported position.

  Different brands of ammo will have different powder loads, primers, bullets, even with the same weight bullet. So POI will be different. That's normal for both commercial and hand loads.
  What was said about scopes/rings/mounts is also very true. The higher the quality the less likely a zero shift.
  Another less obvious cause is method of cleaning. The type of cleaning rod and method of cleaning can make a small imperfection in the muzzle crown. While it may not even be visible to the naked eye think of an imperfection as a speed bump at the crown. Then think how a speed bump affects your car. Imagine what it can do to a bullet traveling 2600 - 4000 FPS!
  Throat and barrel wear can also be cause. As can extreme amounts of rapid firing.