difficult sales

We have a wp plugin which is integrated with many 3rd party services and helps any wp site who captures leads. The plugin had 3 sales and then nothing. There is one negative review on it which was due to the support page being down co-incidentally on the same time when the customer wanted help. Later on when we tried to help, the customer just left it.
anyways, I believe just one negative review cannot completely stop the sales. But any ideas you guys can share generating the sales of the plugin? thanks

Make more plugins.
It gives exposure to old ones.