I am assuming that 3/4 tsb is 3/4 of a tablespoon which should be the correct amount based on a dosage rate of 1/2 teaspoon per gallon.
Did you use the same container of sorbate as the first time? It is possible it is no good or lost it potency.
I also found this info which might help:
Potassium Sorbate
Potassium sorbate does not interact with the residual sugar in any way. Sorbate prevents fermentation by acting on the yeast cells, but it does not kill the yeast. Sorbic acid passes through the cell membrane easily, and the yeast cell is prevented from generating a new bud when enough sorbic acid molecules have accumulated. In other words, sorbic acid prevents yeast cells from reproducing, but it does not stop yeast from fermenting sugar.
Sometimes potassium sorbate does not prevent fermentation from restarting because the wine contains too many viable yeast cells. When potassium sorbate and sugar are added to a wine containing many viable yeast cells, the sorbate prevents the yeast from reproducing, but enough yeast cells may be present all ready to ferment the additional sugar. Then, fermentation often occurs after the wine is bottled. The wine is spoiled, and the winemaker is unhappy.
A different situation exists when sorbate and sugar are added to a well-clarified wine. The clean wine contains very few yeast cells. The sorbic acid prevents the small number of existing yeast cells from reproducing, and the added sugar is unaffected. Even if fermentation occurred, only a tiny amount of the added sugar would be lost because only a few yeast cells are present in the wine. In time, the few active yeast cells in the wine grow old and gradually die. After a few months, the wine contains practically all of the added sugar and very few viable yeast cells.
Using just the right amount of sorbate can be a problem. Wine flavor can be adversely affected when too much sorbate is used, but the wine may restart fermentation if too little sorbate is added. The sorbate needed to keep yeast cells from reproducing depends upon several wine properties. The normal dose is about one gram of sorbate for each gallon of wine. One gram per gallon is equivalent to about 2 level teaspoons per 5 gallons of wine. Wines with a high alcohol content, low pH and 30 to 50 milligrams per liter of free sulfur dioxide present require less potassium sorbate.
An additional problem sometimes develops when potassium sorbate is added to wines low in sulfur dioxide. Large bacterial populations can exist in wines low in sulfur dioxide. Sometimes the bacteria in the wine can react with the sorbic acid, and a strange geranium-like odor is produced.
Large quantities of potassium sorbate were used by commercial wineries to stabilize their off-dry wines before practical sterile filtration equipment became available. However, sorbic acid is not widely used today because of the potential flavor problems and the high cost of potassium sorbate. Now, sterile filtration is the preferred treatment, and few commercial wineries use potassium sorbate.
Potassium sorbate is not a very stable material. Unopened containers have a reasonable shelf life, but after the package is opened, potassium sorbate can deteriorate rapidly. Edited by: masta
