When you're not selling any of the ones you already own.
I'm serious - it's easy to buy domains and very tough to sell them, so if sales are languishing then you need to take a break from buying and put more effort into selling.
That includes keeping your marketplace listings up-to-date, adjusting pricing if necessary, putting some BINs on impending renewals you are not sure of keeping, evaluate any new or unused domain marketplaces, and of course, outbound marketing (in a professional manner).
I used to spend nights going through the auction listings, picking what I wanted to keep track of, marking down my max price, etc., then I went through a bit of a dry spell over the last few months - lots of interest and minimum offers, but zero sales.
So a few weeks ago I decided to split my nights between buying and selling duties. I finally got around to listing some of my domains on DAN.COM, I went back to Afternic and *finally* forwarded them ownership proof of some long-stuck-in-review domains (old owner still had them listed), adjusted some prices, added a few BINs, *finally* replied to some Afternic "leads", and even did an outbound campaign.
And you know what, I sold 2 domains for low 4-figures this week - nothing life-changing, but it is forward movement and I like that. It also proves that you need to put as much (if not more) effort into selling your domains as you do buying them.