Sometimes delays happen, you might need to landscape and other improvements which might just buy you the needed time. 

How can I improve my approach? 

I think the question is more of a personal decision and whether or not someone has the time, energy, and resources available to setup a business, learn the inner-workings of the industry, and constantly reevaluate how you can improve.

Key is to not improve property more than neighborhood.

They got really defensive when I pointed out improvements that needed to be made before it would be inhabitable, and felt like I was just trying to low ball them.

Hey Sean,Refinancing your triplex is an option, assuming either it has appreciated enough or you have forced equity through property improvements.

You can offer to make minor renovations and improvements (helps them and you increase the appeal of the STR).

What improvements/amenities are the biggest bang for your buck an owner can add/do prior to rental?

You need to find HOAs with lower fees and see if the cashflow/ IRR improves in your calculations.

What could they do to improve their situation, even though they are making good money as a sub.Thanks!