Late answer but I forgot to post it after I found out the solution.
The problem was an energy manager in my mobile. I have 3 battery plans available:
- Energy saving - runs only essential services
- Smart - prolongs the battery life by doing some optimization with wake locks, I have had this plan set to active
- Normal - everything runs normally
"Smart" plan was responsible for this issue. As I have read somewhere, when set to active, the system tries to batch wake locks and network tasks, so the mobile is not continuously being waken. Thus my task was being re-scheduled with a longer period, in order to run it with another tasks every - for example - 15 minutes.