How to handle non-paying affiliate programs?

For the past 8 months I've been an affiliate for a good product and they've always paid on time. Well, used to.

Recently they've been very on and off communication and have missed 2 payment dates (payments are monthly). They just keep saying they'll pay soon.

What can you do in this situation other than just keep e-mailing them? They have their own affiliate system so it's not through CJ or any networks.

Is it time to just suck it up and move on?

Fuck it. Rape their inbox

No but if your not losing a ton of money I would just move on.

If you are not losing huge amount, it is better to start with another affiliate program rather than wasting time in the hope that you will get your pay.