How can I fix the wobbly top of my table?

My 12 year old G plan dining room table has recently started to wobble. When cutting up food the whole table wobbles which is quite disconcerting!
We have tightened up all the nuts we can see, but it makes no difference -any ideas please.
Nicola, December 2015


done
Loosen all the nuts you can see, put the table on a dead flat floor and have a teen,youngster,bag of sand,or anything of a few stone placed on it and re-tighter the nuts.It may be out of kilter as they say.

rich, December 2015