Hello!

I'm trying to solder a simple ring made from 4mm x 1.5mm sterling sheet which I have reticulated. But it will not solder!!! Can anyone think of any reason for this? It has now happened with two rings.......(i have succesfully soldered non reticulated rings using the exact same materials in the meantime).
Sterling sheet and hard solder both from Cookson's, I've tried with borax and oroflux, neither work. The solder melts but runs along the top of the ring, one side of the join. The join is very tight but as I heat it it looks like it expands fractionally. Once cool it closes again. Huh?

Any help would be much appreciated : )