could be a long list of reasons. first of all, be sure you have cleansed and charged both the board and the planchette [heart shaped pointer thingie] this could be done a number of ways, use the methods you feel best with, though i suggest using lunar water at some point as the moon rules over divination techniques and lunar water is commonly used in rituals of this nature. once you have cleansed and purify it, place it in a box and/or wrap it up in a cloth until you need to use it. and be sure it is stored safely at all times, damage to the board can bring negative consequences.
before and after using the board be sure to cleanse the area [before so you're circle is in a cleansed space, and after in case anything might be lingering] and of course cast a circle before every use. if you feel something is still in your house, or you're worried, it's a good idea to have a protection charm in your house, and wearing one [even if you don't use a board it's a good idea] be sure to cleanse the board often as it can be bogged down by energy, and spirits can sometimes get attached to it.
now, why the board won't work. first reason it's not cleansed and blessed. in the store people walked by, touched, handled, and moved the board, so it's covered in their energy, you should charge it to work with your energy as well as wash away all other energy on the board. [if your coven is going to use it, you should try and have them all charge the board so it's filled with everyones energy.]
next reason is you're not concentrating. go into a meditative state before using the board. i tend to find you need 3 people to work the board, two to move the planchette [eye closed because they're meditating] and one to ask/record questions. it can be done with one person, but you're mind isn't cleared when you're trying to remember questions and answers. the way your hands are is another way, you should use your index and middle finger, and have them slightly above the planchette, with the fingertips barely touching it. when two or more people use it, you should have your two hands on the rounder end, and the other person in the point [or vice versa] sitting across from one another. [more people you can position your hands wherever, but try to keep the weight equal]
once two or more people get into the mix though, it becomes a 'who could be lying' game, as people do like a good laugh, and could move the planchette just to freak you out. while we're on the topic of logical explanations, slight muscle twitches in your arm, or subconscious desires, can make you move the planchette without realizing. this is why you should have a clear state of mind, and two other people you trust.
the board might also fail to work because you are in a 'dead zone' [don't know if thats the actual term, my covenmate calls it that] where there's not spirit in the area. since you're trying to contact a specific person, they might of been reborn already, or attached to a certain person/place/thing. to increase your chances of speaking with them, you might want to gather objects of theirs, a photo, and perhaps something they were fond of in life [a perfume, or food for example] so you can contact them easier.