Air conditioning

It sounds like you need to have a shop actually test and examine the system. The dye will take a bit to work through the system. But now that it is in it should not be hard too figure out where the leak is. Also, please stop switching shops, the issue is that no shop will generally trust that the other one actually did what they claimed and they each start over.
For a leak in the AC, step one is a visual inspection with a set of gauges. No refrigerant in the system? Pull a vacuum, Does it hold? If yes add dye and charge the system, run the system for a while to circulate the dye. Recheck the charge, and check the lines and fittings with a UV lamp. No leaks found, send it out.
Customer returns with an issue, use the UV to check the evaporator drain and use a sniffer to check as well. If you find dye and the sniffer triggers, replace the evaporator core.

What you want is a good mechanic who does AC work. Does not matter if they are certified or not. Ask around, go on Facebook and ask folks who they like. On of the best AC guys I know works in a corner parking lot with a small shed and a two car tin carport. He has taught me a few tricks that really help.

You could even track the leak yourself. Hit a store and buy a UV flashlight, they are sold all over for pet stains, human "fluids" detection, and blood tracking. WalMart has them, I have seen them in pet stores, parts stores and other places. Get a pair of amber safety glasses that have UV protection. Park in a darkened area and start playing the light over the AC components and look for the dye to light up.
To see if it is an evaporator leak, look toward the bottom of the firewall for the drain that keeps the HVAC box from filling with water, if it is the core you will find dye in the drain or even dripping out of the tube if it is real bad. Expect to find the dye in the low/high side connectors, it gets there when it was injected and when the pressures were tested. Check the condenser core in front of the radiator and the fittings and lines. Might take half an hour to really look.
Look on youtube for videos on AC leaks to see what leaks may look like.
If you find it you would at least have an idea of what it would take to repair it.