How could some joists be 3/4" below the others? That's a big difference.

1) Should I sister up a new joist next to the bowed joists? And if I do, will the load make the current joist sag more?

That could work. Set the new joist with the crown up, clamp them together (flush on top) and bolt or screw them together.

2) Should I flip it and plane what would now be a crown?

no

3) Should I just replace the sagging joists? (Hard for me to do since I can not transport 16' joists)

That could work...crown up. If you can't transport new joists, how can you get sister joists for solution #1)

4) since you have a beam in the middle (not really the middle), you can shim up the low joists.

FIRST...

I would run strings/blocks along the top of the outer joists...front to back, to make sure they are straight.
I also run strings along the sides of the 2 outer joists and straighten them. Use nylon string.
Make 7 identical small blocks of wood, put 1 block between string and joist top at each end of the string lines. Use the 7th block as your gauge. It should just fit between string and joist top...everywhere.
I would run 3 tight string lines across all of the joists...one on each end, and 1 in the middle.

Tweak the joists by shimming/planing until that block fits right.
When you're done with the strings, your deck will be flat. Not necessarily level, but flat, and that's what you need to achieve.

steve

Steve:

Thanks for the response. Regarding the joists to sister up to the bowed joists, I can carry up to 10' joists. Most of the joists that are bowed tend to only be bowed in a 5' section or less. So I would not be sistering the whole length of the 16' board. I think this is what I am going to do.

I did use the strings/blocks on the whole deck and that is how I found out several of the joists were bowed. I have been driving myself crazy trying to "level" the deck. I am going to work today and see if I can get it "flat" as you said instead of "level".

With my old pressure treated decking, it looked like a rollercoaster in several spots because of the sagging joists. I don't want my new cedar boards to look this way especially since I am using a hidden fastner system.

Thanks for your help. Will post outcome here.

Gene

Gene,
A 10' sister on a 16' joist is just a step-sister. No real relation to you. Your call.
I would install a solid beam across (under) the middle of the joists, and shim/screw to that...solid...no bounce.

Hidden fasteners? good luck with that.

It's your deck. You get one shot to do it right.
Unless you want to do it again and again.

steve