Are all vitamins/supplements the same?

I usually take biotin for hair skin and nails and I buy it from walmart. The last time I bought it from the dollar store and I couldn't tell I was taking it. When I was getting it from walmart I noticed it with my hair skin and nails but I didn't notice anything with the dollar store kind. I also got b12 from the dollar store. My friend takes b12 and she said it gave her all kinds of energy and made her feel great. But with the kind I got from the dollar store I haven't noticed anything at all since I began taking them. What would the difference be, if any between the vitamins and supplements from walmart or gnc and the kind from the dollar store?? It seems like I wasted my money on the kind from the dollar store:(