without further information is hard to tell where you are going wrong but here a just a few questions to consider
Did you do these nails for cheap / discounted rate? If yes then clients will never look after something they see no value in
Were the nails in bad condition i.e peeling , thinned out , bitten etc ? Clients with challenging nails cannot expect 2 week wear time out of shellac , they need to work with you with their aftercare to ensure that they get their nails into the best shape possible and then they can expect 2 week wear time out of them.
Are you using the entire system as a whole, I.e all CND prep products, CND base , colour and top coat and CND lamp? If not this could be why, you need to use the system as a whole