Do me a favor and explain what "a Chlorine Lock/Demand issue" is.
In reality it is a buzz word that pool stores use that means absolutely nothing.
Your water is clear and something is making your FC drop right? Since your CYA is not zero, it is likely do to something in the pool breaking it down (you could do an
Overnight FC Loss Test to confirm that something is in the pool) and not the sun.
In reality, what you just described is ALMOST the shock process. You have to just keep adding FC until you have consumed whatever is in your pool. The process is exactly the same for a green swamp, a problem with ammonia, and whatever your issue it.
If you would like further info on what we would suggest you do, post up a full set of test results.
At some point you need to decide if you are going to continue to trust the pool store to spend you money wisely, or if you would like to take control and understand you pool and spend less money leaving you more for refreshing beverages.
For example, why would they recommend the MOST expensive chlorine product out there (lithium hypo)? Think it might be to make money?
My recommendation:
1. Stop going to the pool store
2. Order one of the good recommended test kits
3. Read Pool School a few times
4. When you get your kit post a full set of results and then go through the shock PROCESS
5. Enjoy the tasty beverages you bought with your saved money by your sparkling pool.