What weeds are they? you may need a different weed killer? I find I really need to clear the weeds before putting down weed suppressant material and then I use 2 layers of it under a 4 " of chosen covering.

I'm wondering what terram is, never heard of it - is it a trade name for a geotextile?

ive never herd of it either . the thing with mulch and membrane is eventualy you do get build up of enough soil to grow weeds on top of it and worms bring soil up to . it just mite need redoing and as bamboo sais it just mite not have a deep enough mulch on it .

a lot of salt works well as a weed killer if you havnt got other plants there . it looks like snow for a while but eventualy melts threw into the soil . you can buy big bags from a central heating supplier .

Bamboo
I think terram is a menbrane which is use on building sites which they put down first before putting down stone/concrete etc roads/driveways etc

thats know good as its not poures if so as all your mulch will slipof it and the polythene will break down in the sun hence theweeds etc .

A geotextile then, presumably, just heavier duty than the bog standard weed membrane, Jiffy?
I'd ask how long its been down with the chippings on top of it - Nosey's right in that soil gradually ends up on the chippings and weeds do germinate in it. Unless the questioner means the weeds have penetrated the membrane and are appearing anyway - Japanese Knotweed would do this.

Couch grass will just push its way through almost anything except concrete and I am not so sure about that either. We have weed suppressing membrane on various gravelled paths and I still have to use Pathclear on them ever few months. Life has a habit of living.

So will Crocosmia I noticed it coming through and when I lifted the material they were doing very nicely thank-you.