Shellac is designed for healthy strong natural nails. If there are any problems with your nails then you cannot expect shellac to last the full 2weeks.
When I first started out with shellac I was getting 2 weeks out of it easily. I then went onto a medication which affected my nails, because of this I was lucky to get a week before it started to chip etc. It took a while after I stopped the medication for my nails to get back to their original condition but now they are I can get the 2 weeks wear out of it again.
I agree with what souz has said though, as a tech our nails are in a lot of chemicals which can break down the shellac. So we need to take as much care when using them as possible so we don't damage our shellac. I've done it before but soon got into the hang of how to make sure my nails stayed away from the acetone etc.
If you think you need extra strength to you nails the fibreglass trick is a good one. Fingernailfixer on Facebook and you tube has a video on how to do it.
Here are the steps:
PEP nail
Size fibreglass and apply to nail trying to handle it as little as possible.
Cut fibreglass down as much as possible to free edge but leave a few mm of over hang.
Scrub base coat into the fibreglass and cure for 10 secs.
Carry on with colour coats and top coat curing each layer for 2 minutes. Obviously you can cap edge as normal but don't worry.
Wipe off inhibition layer then file down the bits of fibreglass that are poking down so mail is shaped as required.
Wipe with dsperse again to remove any dust on nail.
Apply a second top coat making sure you cap the edge this time.
Cure and wipe nails and they are done.
The fibreglass will soak off within the 10mins that it normally takes to remove shellac.
HTH xx