The most common thing to fail on a Roman blind is the cord.
A blind that is used daily may well need re-cording each year.
We have found that the best cord to use is a low friction Roman blind cord. This is a 1.2 millimetre diameter braided nylon cord.
People make the mistake of thinking that thicker cords would be stronger and last longer. They in fact cause more friction and wear through more quickly. Cords thinner than 1.2 millimeter just cut into your blind.
There are many cords out there that are listed as Roman blind cord but are unsuitable for the job.