Suggestions on encouraging my Kribs to spawn.

I have a pair of Kribs who have coupled up and have found a cave to nest.
Every couple of weeks they take it in turns to dig out the subtrate in there mouths then the female sits in the cave for a few days while the male chases of other fish.
Then nothing no eggs in the cave or fry.
Then a couple of weeks later the same happens again.
How can I encourage them to lay eggs and fertilise them, I've tried giving bloodworms, turn the temperature up a degree or two not sure what else to do any suggestions will be appreciated.

Have you got a male and female or 2 females?
Post pictures of the adults so we can sex them.

How often do you feed them?
You want to feed them 3-5 times a day when trying to breed them.

How often do you do water changes and how much do you change?
Big (75%) water changes done every day or two can help get fish going.
Make sure any new water is free of chlorine/ chloramine before it's added to the tank.

Chances are they're laying eggs. The pattern you describe means either infertility, or that your water is wrong. If it's too hard, the larvae will die. They should be at 26 or so, not warmer.

Male dwarf Cichlid infertility in appropriate water sometimes disappears with age. It can take a few tries.

The female doesn't seem to be laying any eggs though as I never seem to see any in the cave for the male to fertilise.

Totally agree with @Colin_T . The big water change always works especially if it coincides with a new moon .

If you lift the cave, the eggs are doomed. I've bred lots of kribs and their relatives without seeing the eggs or wrigglers.

The female will eat any bad eggs to protect the others, and if they're all bad...

I agree that the best trigger is the water change at a full moon, or even better, right before or during a thunderstorm in summer.

I have a pair of Parananochromis who spawn every 2 weeks, and guard for 3 to 5 days before they give up. I am working my way through reasons. I keep bringing down the water hardness, by 10ppm tds every time. I'm also increasing flow around the nest sites.

kribs should be a lot easier. But they don't read the rule books.

So they have really dug out the cave a left the biggest mound yet, however the male is in the cave guarding it and won't come out, while the female just hangs out at the entrance.
What is happening here?

Well that didn't last long, they are now taking it in turns to go in and out of the cave when not hanging round the entrance.