Too Small - As a general rule the a tattoo that needs to be "covered up " by another design will have to larger than the original piece. If the original tattoo is 2 inches by 2 inches than the cover could no less than 4 inches by 4 inches and ideally would be much larger. Making the new tattoo design larger than the old one allows the artist room in the new design to hide the old tattoo with out losing the integrity of the new design.Too Light - Tattoo inks are applied in a scale from light to dark. While tattooing a darker color can stain a lighter color, so the tattoo artist while coloring the tattoo will begin by doing all the dark colors first than the light colors. This also applies to the tattoo cover up, a tattooed color can only be covered by a darker color i.e if you have a red tattoo it will need to be covered by black or blue, not yellow or orange. Choosing the right colors is a big part of getting a cover up that will look good after it has healed.
Too Detailed - Probably the biggest complaint about finished cover up tattoos is that you can still see the original tattoo threw the new design. Some of the reason this happens is the design chosen as a cover up was to detailed from the start. The idea with a cover up is to hide the original design while keeping the integrity of the new design. One of the ways this can be done is scaling back on the detail of the new tattoo, replacing detailed line work with thick lines and bold fields of color will help cover the original design while keeping the new tattoo clear and legible.