The coating does not hide the surface as expected.
Insufficient topcoat has been applied.
Topcoat has been over polished allowing the coating below or substrate to shine through.
Top coat has not been properly stirred.
Make sure products are not being overthinned and are being applied to the recommended film thickness.
Take great care polishing, especially where paintwork follows contours and grooves, (such as car detailing).
Make sure products are thoroughly stirred before application. Metallics in particular can have lowered opacity if not adequately stirred.
See "bleeding" troubleshooting article.
Some colours naturally have better hiding power than others, (black hides better than yellow). It can sometime be advisable for ease and cost to lay down a tinted primer first then the topcoat to achieve the end colour.
Apply additional coats, (providing the system allows for this). Some systems, (such as fully cured two packs) will require sanding and degreasing before additional coats can be applied.