We had a similar situation at home - one burner suddenly started lagging. Turned out the problem was in the contacts: they oxidized, and the electrical resistance increased, which caused the power to drop. If your stove has been running for a year and a half and was fine before, then it's most likely that.
You can try something simple first: clean the connection between the burner and the socket really well. Turn off the stove, let it cool down, then pull out the problematic burner and look at the contacts - they might be dark or have buildup on them. Gently wipe them with a dry cloth or soft brush, then put the burner back in and check. If there was dirt, it could be preventing proper contact.
If cleaning doesn't help, then most likely you'll need to replace the burner itself - the heating element inside might wear out or get damaged. It's not a very expensive part, and this kind of repair is pretty affordable. I wouldn't recommend replacing it yourself if you don't have experience - it's better to call a technician so you don't damage the socket in the stove.