I agree with Morgan, it's probably the thread.
There are four basic ways that metallic thread is made.
1, Very fine strips of metallised polyester is twisted together with one or more plies of silk, rayon, nylon or whatever in much the same way as regular sewing machine thread is.
2, A core of core of rayon/nylon/polyester thread is wrapped with the strip of metallised polyester.
3, Strips of the metallised polyester and plies of another fibre are bound together by 2 ultra fine filaments of polyester, one filament goes clockwise, the other goes anti-clockwise.
4, A mix of metalised poyester strip and other fibres are chained together like a long length of crochet chain
1, Tends to be stronger but is sometimes thick and stiff.
2, Probably the shiniest but most likely to shred because the metallised polyester wraping is quite fragile.
3, Usually quite good for machine embroidery.
4, Generally quite thick so not suitable for machine sewing.
If your Hemline metallic is
this then it's a wrapped thread.
My advice would be to look for a different thread, probably a twisted one such as
Madeira Supertwist or
Superior Threads Metallic or
Coats Ophir.
Or
YLI Candelite if you want something a bit thicker.