Hi @Santi,
As @kimanh.lecao mentioned it would be helpful to know which version you are using, and if you have tried updating to the latest Bioconductor version.
Additionally, please make sure the sample names are standard characters without any special characters (space, !@#$() etc.). You can paste here the output of the following code:
lapply(X, rownames)
for us to try and diagnose the issue.
You can also simply send us your data following step (5) at Reproducible example to clarify issues for us to spot the problem.
Best,
Al