SpectroVis spectrophotometer values changing after pulling cuvette out

I am required to write a research paper for school in a specific subject and I have chosen to write mine in chemistry.
I am investigating the kinetics of the Briggs-Rauscher oscillating reaction.
I am using a spectrophotometer to take samples of the reactants and determine the absorbance at 460nm every 45 seconds to calculate the rate at which iodine is produced.
However, I am using a spectrovis spectrophotometer, who's calibration values change without me doing anything and sometimes do not register the fact that iodine concentration has increased. Is there anyway I can fix this problem and prevent the readings from changing?


