Witchipedia

Take my word for it: you don't want a love spell to work. If you tried and failed then consider yourself lucky and just don't try again.

There are love spells and there are love spells. There are many reasons a love spell might not work and they really depend on what you're trying to do. If you put a spell on yourself to attract love, you have to really focus on making yourself love-able. You have to put love out, to get love in. And if you're doing all that and still not seeing results it could just be that it is working, but you're not seeing results yet. After all, things have to fall into place and that can take time.

If you've put a spell on someone else in hopes of forcing them to love you, there are several reasons why this might not work too. Spells only work if there's a reasonable chance of that thing happening in the first place. They are a nudge. You can cause a person to have lustful dreams about you easily enough. But if that person already finds you repulsive, they aren't going to wake up wanting to run into your arms, they are going to wake up wanting to take a hot shower with bleach.

If you're hoping to just make someone run into your arms, again, this depends on how they feel about you in the first place and what other emotional entanglements they have and what life decisions they have already made. If someone has firmly decided that they don't have the time for a relationship right now, you have to do more than bend their emotions, you have to make them rethink their whole life. If someone is thoroughly in love with someone else, and loyal, it's also not going to work on them (though it might make them feel very upset and confused)- You'd have to alter their whole personality. And to further complicate things, if you want to change someone's whole life plan, or alter their personality, your Will must be much much strong than theirs (and you must also have very little regard for them as a person. Sociopathy is helpful here, but I'm not sure that's something you can turn on and off.). If their Will is stronger than yours, you are out of luck entirely.

Basically, love spells work best on people who are already lonely and perhaps have low self-esteem or maybe are unhappy with their current situation and have no concrete plans for the future. There are plenty of predators out there who know how to take advantage of people in that unpleasant place without magick.

(although to be fair, love spells also work on people who are maybe interested in you, but haven't quite decided or worked up the nerve to take the plunge and also people who you know are into you, but have been distracted lately. Magical spouses occasionally cast love spells on each other- between consenting adults, but candles and soft music also works wonders.)

Long story short if it's a generic spell (e.g. "o universe please bring love into my life") then just do what you think needs to be done and send out the energy and it should work.
If it's a specific spell (e.g. "o universe make John/Jane Doe love me") then just don't do it to begin with.
1- it's rude and unethical to try to control people like that
2- people have free will, if you send out a spell like that they can resist it and your spell was pointless

A SLIGHTLY more ethical idea might be "o universe, make John/Jane Doe notice me more"
That way they still have free will and if they have feelings for you they can come out naturally.
But BEWARE if the person doesn't know you (like someone famous) it will probably not work, or if they don't like you, you'll just be irritating them/they might notice you in a bad/embarrassing/mean way.

I did do the first kind of spell I mentioned though and less than a month later I started dating my current girlfriend of 1 year and 10 months.