Adhesive
Make sure to test if the surface on which you plan to stick the adhesive side of the magnets is adhesive-friendly. This can be done by applying any type of sticker and seeing if it sticks well. Examples of surfaces that are not adhesive friendly: clothing, wool, and beads.
Magnetic surface
Make sure to test if the surface on which you plan to stick the magnetic side of the magnets is magnetically receptive. This can be done by testing it out with any type of magnet. If that magnet sticks well to the surface it's magnetically receptive. Materials which are made up of aluminum or copper (and not iron) are not magnetically receptive for example.
Weight
Our rubber magnets are designed to hold light objects such as but not limited to postcards, business cards and small artwork in a cost-effective way. Anything flat weighing 10 grams per attached magnet should not be a problem. For heavier items such as but not limited to whiteboards, blackboards and key racks, we recommend buyers to buy other magnets.