It really depends on what the issue is for the buyer. Try asking the buyer directly why she doesn’t place an order if she actually wants your service. Some buyers really don’t understand that they have to order first. For those, a careful explanation and maybe even a screen shot of the order button on your gig can help.
There are unfortunately some buyers who will use the message system to try to get around paying for their gig. They hope that if they send their files and push you a little, you’ll give them the work first. Some will even promise to pay after delivery and claim they are waiting on payday or something. The problem is, many of those disappear once they get delivery, so don’t let anyone push you into that. If they don’t order after a nice explanation, you may just have to let it go. I probably get 1 in 10 that say they are planning to order for sure and they never do.