OD660 readings of yeast
Many inexpensive spectrophotometers cannot measure absorbances over 1.5 or so reliably. It's possible that you're out of the measurement range of your instrument. Try diluting your samples with water first. If you dilute 1:1, then the absorbance value of your sample is double what you measure (assuming the Beer-Lambert law is valid in this situation)

Also, I imagine molasses is very absorbent at that wavelength so it might be obscuring any change from the yeast and it may not be possible to measure the yeast growth by absorbance, with such a matrix.