I have been working with real estate agents, and sending out letters to home owners, with pretty good results, however after i put an offer on a few places and got turned down my agent said my offers need to show proof of funds to be considered stronger.

Evened an assistant to a transaction coordinator will see lots of offers and terms and tactics employed.You can offer a higher price then peck away before closing escrow, make a stronger offer by increasing your down, reducing or eliminating buyer credits, and show stronger financials, if possible. 

I offer all cash - in immediate escrow, immediate closing, no contingencies - if there is a stronger offer please tell me what it is.

You will have to look at more deals to find one that works, but the properties you buy will be better/stronger investments.I will add that I don't have a different LLC for every property I own.  

I also want to do some work on the place to increase the under market rents, which will show a stronger rent history should we decide to go for a construction loan to add another unit at refi time.Using your scenario, I'm also assuming they would retain ownership of the property, which could put us in the sticky situation of putting money into a property we do not own--something I am not comfortable with.

It may make you feel better or help you choose which buyer but it does not make a contract any stronger

There will be growing pains but if you keep an open mind and a willing heart to look out for your private money's best interest they will reciprocate and want to continue to grow better and stronger with you.

What usually attracts people to those markets is the low prices but for $75-$90K you can find much stronger markets.

Awesome to see this effort is still going and even stronger!  

Going from $400 to $600 is likely to produce a much stronger reaction than from $2000 to $2200.What's the demand like?