My car blew warm air after the indicator jump to red zone, then I took it to the repair shop, they?

changed the thermostat , as soon as next day, my air conditioner blew warm air when I want cold air, then I added in 1 can 14 oz.ez freeze R_134a but still blow warm air as before, should I tried to add in 1 more can before I take it to the repair shop, because, under the hood the indicator show it need 2.5 lb, but they doesn't it get a bit cooler then before, I use the soap and water to check if there is any leakage, but doesn't look like there is any leakage at all. Please help.