Forced carbonated, beer flat

I just kegged a brown ale a few days ago, cooled it down to 40 degrees then force carb’d following a chart @11 psi. I was aiming for 2.3-2.4 volumes. I shook the keg for awhile until I couldn’t hear bubbles anymore. Maybe the co2 tank is almost empty? The gauge on the left side of the regulator is at 750.

If your tank gauge reads 750, it’s not empty.
I find that it takes 7-10 days for a keg to be fully carbed.
Shaking the keg will speed that process up, of course.
Not sure how long the keg has been hooked to the gas but after a few days (3-4) there should be some signs of carbonation.
You may have a small leak somewhere. The first place I check is the keg lid.

Okay thanks I will start with the lid.

I agree with Brotillery. It’s too soon. What’s the hurry? I would leave it alone for at least a week.

Your pressure was too low.  I get good results with 30 lbs.  i sit in a chair and roll the keg  gently back and forth.  After 7 or 8 minutes I get around 2.3 volumes.

I agree but I start at 20psi instead of 30, because at 30 I have overshot carb level!  Believe me it’s easier to get CO2 in than out.  I leave it till the next day with the gas disconnected and then finish off by shaking the keg with the gas set at my desired final psi.  It’s still a lot faster than doing the whole job by just sitting there under serving pressure.

Agree with Buzzsaw.  What’s the hurry?  That beer needs to age a bit and mellow out.  Give it two weeks and it will be killer beer.

^^^^
If there’s still yeast present, it might age and mellow a bit, except that as you approach your target the pressure will send the yeast into dormancy.  I filter before I force carbonate, and if you do this you want to get it carbonated and on tap as fast as you can – once the beer is separated from the yeast or the yeast goes dormant, the beer is not aging. It’s just getting old.