To become more clairvoyant/etc, you need to work on your senses. Through meditation, scanning (practice), working with spirits in general will work out your senses over time, divination can also help, etc.
Not sure what method your using to call the demon, and not familiar with that particular petition ritual (I’m familiar with the one from @DarkestKnight) but if you use the sigil of the demon (and open it successfully) then there’s a good chance they heard you/are present EVEN IF you can’t sense them.
One more tip, from what I know - love spells only really work if there is some base to build from. If this ex has no more feelings for you then most likely won’t be successful. There’s needs to be a base from what I hear. Kinda like a foundation to build on.
This is the petition spell I’m referring to: