Hmm annoying, but honestly, you barely need to be able to sew to tack up a trouser hem. Just turn them inside out, thread a needle, and loop it through in a diagonal fashion, with small stiches on the good side, and big stitches on the other side. Or running stitch (stitching in a straight line) very similar - big stitches on the back, small stitches on the front.
I can't view Youtube, but aparently here is a video showing you how to: here
If you take them back to a store that does alterations (usually in the mens dept) and speak to customer services they may offer to re-hem them.
The reason the hems don't take is because of the polyester fabric and the fact that when they are machine stitched it shows on the outside of the trouser. This shows a certain ammount of care from the manufacturer, but can mean that the hems aren't as secure as they could be.
Having said that, how lazy do you have to be to not try to mend them yourself. It would certainly take less time than returning them to the shop!
My DDs hem on her MandS dress came undone on second day of wearing - I hemmed the part that's come undone and now the rest has gone - what a load of crap - I'm goingback to Asda next year DDs dresses lasted all year- only not using as shes grown
Tassissss - you can usually see a small line of stitches (even with machine-done hems), but you can usually get it fairly small by doing big stitches on the back and small stitches on the front, and by using thread as close to the colour of the fabric as possible. It always looks a million times better once you've pressed it.
NAGY does make an interesting point re sewing in schools. It used to be part of what you learned when I was at school - for both boys and girls. I was just thinking the other day that when I was DD1's age (less than 30 years ago ) we were doing cross stitch in class. Seems very arcane now - but also very useful!