Geeg has posted in the Shellac Lovers group detailing about how Shellac CANNOT be compared to any other coloured UV polish on the market. Shellac is one of a kind and is NOT a Gel. All these other UV polishes are. So that is the first thing, don't compare Shellac to any other service as it is unique and completely different and is not used in the same way.
As far as not getting more than 5 days wear out of your Shellac, then you need to go through your steps to determine where you might be going wrong. Like others have said above, are you using the whole system? Are you using the correct lamp? Are you doing a thorough PEP ensuring there is no cuticle on the nail plate and are you thoroughly "scrubbing" your nail plates with scrubfresh before application?
The application itself needs to go on just like a polish, thin to win. The first layer should look streaky, the second should look perfect. And you need to make sure you are sealing the free edge, not capping, with each layer.
How many clients are experiencing problems? If it is several, then it is something to do with your PEP or application as to why this isn't lasting more than 5 days. If it has only been 1 or 2, then it may be that Shellac just isn't suitable for them. It's rare but there are some people who cannot wear Shellac due to the condition of their natural nails.
Maybe if you talk us through your steps we can help pick up why this may be going wrong for you :hug: