The following could be the reason:

• Incorrect roller used for the specific surface. When purchasing a paint brush or roller, read the directions on the label as this will direct you as to which area should be painted with that specific roller. 
• Certain rollers such as foam rollers roll a thinner coat of paint than other rollers. 
• The paint is not applied correctly: When painting with a roller, it is easy to roll the paint out too thinly. This will result in the coats not covering. This is called stretching the paint. 
• When repainting a dark coloured wall with a much lighter colour, you may need more than two or three coats of the lighter colour before the darker colour will cover. The use of a primer might be beneficial as this will reduce the number of top coats that will be required.