The ability to use magick and cast spells can depend on many things, knowledge, practice, state of mind, emotions, surroundings etc...the best rule of thumb for the beginner or novice is to take all these things into consideration.
Try to find a calm secluded spot where you feel at ease and able to work without interruption, then take a few minutes to prepare yourself. Calm the body by regulating the breathing and allowing muscles to relax. Slow the mind down and focus on the spell you are casting. Think of the emotion that the spell involves...Love, Loneliness (missing someone), Happiness (trying to lift someone’s spirits) etc. Whatever spell you are casting...find the emotion associated with the spell and begin to focus and build that emotion inside yourself.
Send that emotion out...into the astral/spiritual/psychic world...as you work your spell. Whether it is incense, candle magic or a technical ritual ceremony, you will always achieve better results when you can concentrate and focus the emotion behind a spell or ritual.
Spell casting, like all things in life, takes practice, but you also have to learn and know "what to look for" when you cast a spell.
For example, if you cast a love spell...the person you cast it on may come take you in their arms...and they may not. But if done properly...be assured that at the moment the spell was cast...the person thought of you, in some way or another. And, most likely...you will also find 'something' occurred...phone calls from "admirers", romantic advances, love letters...any number of things related to love can occur when a love spell is cast. Learn what to look for...after casting a spell, and you will understand better how they work and what to expect from them. This in turn, will help you strengthen and improve them. In time, as you practice your abilities you will learn to control exactly what the spell does.