Blockages are usually caused by something that has gone down the plug hole. This could simply be a build up of products or hair or cooking fat
It is tenants responsibility to keep the drains flowing in the property so firstly you should be careful about what you let down the drains
You can purchase 'drain un-blocker' from any local supermarket. Follow the instructions carefully. Do not just use bleach products and drain un-blocker has the specific job of expanding and breaking down and build up in the pipes.
If this fails to work after a couple of tries, then you should inform Unipol, but please be aware that the cost of unblocking drains is usually passed on to tenants unless there is a problem with the pipes (and this is quite a rare occurrence)