I like Asteriskos' warning that one must be sure what their intent is, and I think that's the cause of a lot of apparent failures of love spells. I reflect a lot on what Plotinus says: magic may affect the passions, but never the reason.
One can cast love spells and infuse infatuation, but if on an intellectual level they know they don't want to pursue this, then they're not. If the lovers are incompatible, perhaps at best you can hope the target will declare they're dating in a heat of passion, and then have to live with the reality of what that means. Sometimes it goes over great! A love spell I casted for a client resulted in a marriage, woohoo, they already had good potential for chemistry. But sometimes not.
I have a friend who repeatedly casted love spells on this one guy, and I must admit, they love each other. But love itself isn't a permanent bond. She keeps manifesting these cute moments where they bare their heart to each other and declare their undying passions, but it never solidified into a relationship again. The energy of the spell runs out, and the guy tells himself despite his love he--on an intellectual level--reasons he doesn't want or can't have a relationship, so they drift apart again.
I think it's part of this cultural meme of love spells turning to obsession spells, because if the target can't articulate any reason to want to stay together, the only thing you can hope for is inflaming their passions enough to override their sense.
This isn't to say love spells are doomed, I really believe magic can change any situation, it just comes down to what tools you use. There is a massive diversity of 'love' spells out there; 'love' in quotations because so many things can fall under it. One can provoke love via an erotic binding spell (to use Sledge's terminology), one can create a love spell by withering a cheating partner's desires for other people, and so on.
I co-sign the idea that love spells can be beneficial for preexisting couples. By naturally becoming couples in the first place, it's proof of an existing chemistry, so there are plenty of avenues of permanent manifestation by helping them understand what caused them to get together in the first place.