My first build. Everything is great but the Slide is not locking back on the last round when I fire it, will lock on all my mags when I rack the bolt .  One thing I did see is when the upper is off the hammer touches the slide stop. What is the problem? Thanks

Read the tacked threads in troubleshooting.  You have a gas problem.

What ammo are you shooting?

Could be a gas problem (short stroking) or a magazine that's not functioning properly.

1.  Clean and lube weapon per the TM with CLP.

2.  Shoot quality known good ammo to test with.  Assuming you have a NATO or Wylde chamber, and not some SAAMI match, use Q3131, Q3131A, Federal XM193/855, AE223, etc.  Dont use Wolf, Monarch, or Barnaul for reliability testing.

3.  Insert one round in mag.  Chamber, and fire.  Bolt should lock back.  If it doesnt - you most likely have a gas leak, or binding.

Ensure you have all the correct components.  Correct buffer, buffer spring, etc.  Ensure your bolt catch works 100% when manually pulling CH back.

Work action back fully and feel for any rubbing or binding.  Inspect upper receiver, carrier, and bubbfer tube for signs of rubbing/wear.

#1 gas leak/short stroke area is the carrier key.  Ensure the key screws are tight, and staked.  These get loose over time if not staked properly.  Inspect gas rings on bolt - insure you have three installed.

Lastly - describe your rifle in detail.  Type of stock, buffer, carrier, barrel, etc...  and mags you are using.

It could be several things...
Inadequate gas pressure due to:

Leaky or misaligned gas block
Loose carrier key

Or it could be from a tight chamber. If your chamber is tight, it takes so much energy to pull the spent case that there isn't enough left for your carrier to slide far enough back to catch.

I've seen guys shooting their AR's without the stock even touching their shoulder.
This caused a short-stroking malfunction. Make sure to hold it in tight!
N2OGV

Quoted: I've seen guys shooting their AR's without the stock even touching their shoulder. This caused a short-stroking malfunction. Make sure to hold it in tight! N2OGV

I've done a lot of shooting in wierd positions.  Never shortstroked when the stock was off my shoulder.  Ever.

And if your AR has a slide, it must be a new model.  Mine has a bolt carrier.