Hi pierre3480 – it would be worth figuring out in the first instance what is causing this to happen (as it does not happen on a “plain vanilla” installation, for instance).
Can you try:
* Deactivating all other plugins except for those by Modern Tribe
* At the same time, switch to a default and unmodified theme such as Twenty Twelve
* Check if the problem is still present
* Assuming it is not, reactivate your theme and each plugin one-at-a-time and test at each stage to see when the problem returns
This should help to isolate whatever is causing the problem – and we can then potentially zero in on possible solutions. Thanks!