There's a difference between polish, which will clean surface paintwork or grp but is slightly abrasive and and wax, which will seal a surface leaving a durable shine.
Polishes leave a shine because polymers are part of the composition but repeated polishing if the abrasive content is too high could dull grp surfaces.
There is no magic formula available to restore the shine so the best thing to do would be to apply a coat of wax ( preferably a product which contains carnauba wax) over the polished surface and continue to buff and apply successive wax coats until the shine is restored. The more work that is put in, the better the result.
I normally use
Meguiars stage 3 wax (available from Halfords) will should leave a shine which should last for about 3 months but any good quality automotive wax should be fine as long as it is not abrasive.