The most likely reason is that there is a voltage drop due to the total cable length being too long. Please double-check that you do not have too many light strings in a row as this varies between 2-6 maximum depending on the product. You can find this information on the product's packaging or at www.lightson.se

The solution if rthere is less light than usual is to split your lighting across several transformers.

Of course, it could also be that the string is defective. You can easily test this by connecting it directly to the transformer. If it lights up as it should, then the string is intact.