Strobes only work on Cloudy days?

Ok, I bought some cheap chinese A/C Strobes about 2 years ago.

They have worked fine up until today.

I had them setup on the back porch taking shots of birds near the feeder.

It was cloudy today, and the strobes worked fine with a hardwire sync.

When sun came out, they quit working.

I checked the power, the test button the camera, the test button the strobes themselves. Everything seemed to fine, but they just stopped working after taking about 200 shots.

I unplugged them, power cycled them, nothing.

I broke down the equipemment moved them back into the garage, and tried them and they both worked fine in the garage were it is fairly dark.

So, is there something I'm missing here? They only work in bad light?

Please don't tell me its because they are cheap chinese products, that can't be the reason.

thanks.