Syncing iMac and iPhone using Notes

I had been using Notes on both my iMac and my iPhone (as well as my iPad occasionally) - especially for shopping lists. At home I would write my list and it would appear on my iPhone, however, once I upgraded my software, it no longer syncs between devices. I've checked ICloud and both devices are on and supposedly are using that, but still they are not talking to one another. What should I do? Thanks.