No love spells are guaranteed to work, and there is no reliable evidence that any love spells are effective. Even psychics themselves warn that no psychic is always successful, which is a good indication of the unreliability of magic and spiritualism. People who claim to be able to cast love spells usually offer anecdotal evidence of their efficacy that should not be considered good evidence.
Most love spells are based on tradition, superstition and guesswork, and the evidence for their efficacy is not scientific. Spells probably work in the same way a placebo does. A placebo is an ineffective treatment scientists use in tests, and studies show that placebos usually produce a positive effect. More surprisingly, placebos can be effective even when patients know they aren't real treatments. A spell might give a person confidence to achieve their desires and a sense of control over their destiny, but that doesn't mean that the spell actually works.
Scientists who study intimacy and attraction have identified several ways feelings of love can be stimulated, even between strangers: gazing into each others' eyes, physical nearness and sharing secrets. Other ways to stimulate love between partners include sharing new or exciting experiences, being kind and forgiving to each other and sharing humor. Any situation which allows vulnerability or builds trust can be positive. It is important to understand that both parties enjoy free will, and nobody can be forced to love another person.