someone said (i'm paraphrasing): A/B on the laptop with a pro mix. i will add to that:
1. find a pro mix in the same genre as your music, and a song similar to yours. i.e., avoid that limited edition hannah montana movie soundtrack if your song is heavy metal.
2. give your ears a break before listening.
3. level match - if your mix is quieter than the pro mix then turn the pro mix down until they are the same apparent volume.
if the pro mix sounds good, and yours does not, then there is a problem with your mix. could be a million different things, but likely to be in the following broad categories:
level matching between instruments - very unlikely that your laptop speakers can be driven as loud as your headphones/monitors. turn down your mix in headphones/monitors/car and see if certain tracks pop up.
frequency issues - the laptop acts like a bandpass filter. try putting one on your mix and see if you get similar results. if a lot of your instruments are bass-heavy (guitars, bass, kick, etc.) this could be the problem.
stereo imaging - depending on the laptop, it may have pretty poor imaging which is causing some of your wider instruments (think fast delays, doubled tracks, anything in stereo, etc.) to thin out vs. centered/mono tracks. the quick way to find out is to put your mix in M/S (mid-side) mode or mono and see what happens. there are plugins for M/S and you can just mono the final output bus (A/B both with the stereo mix).
hopefully, the underlying cause is that your laptop has rubbish speakers but if you've never mixed at low levels i'd put that at the top of the list.