Last-ditch is to always keep on the hunt for more potential investors. In an ideal world these investors would be competing against one another for your project. Lame answer but true.
On the more creative side would be to create some energy or some excitement around the project as almost a takeaway. There are many folks who will tell white lies in this direction which I cannot promote so think being honest is the most sincere way to generate a response without imposing on them. It also helps in creating a lasting relationship as you move on to other projects, which is another enticement. The idea of duplicating your process.
That being said, you reiterate numbers that you can accurately portray and stress your worry as you do here in potentially losing the investment opportunity. "Let's put in a lower offer and see what he says. We already agreed the value was there." The other is to show actual numbers that you see. "There are this many people looking at this every day. Right now. There are this many offers. I know you are busy and we have plans but we have to make a move or will lose it."
Take the ideas for what they are and I hope they help.