flash diffuser - overexposed

[hungrymark]

I was using a flash diffuser on a Sigma EF 530 DG Super the other day for a bit of macro work and every image I took came out massively over-exposed. Took the diffuser off and the exposure was perfect. I've used it before using a diffuser to no ill-effect. I was using auto TTL metering (flash) in P mode on the camera (Sony A550).

Does anybody know what was going on?

 

[Matt]

got any example shots?

 

[hungrymark]

No, I realise that would have been helpful but I deleted them because they were rubbish :bonk: