Decreasing signals in assay measurements
Freeze-thaw cycles are often suspected of causing degradation in organic molecules 1,2. My first guess would be that your fluorophore is breaking down due to those repeated cycles. Alternatively, you might be getting precipitation of a calcium compound from the buffer you are freezing that your colleague is not.
Standard laboratory practice is to make aliquots of frozen solutions. Before thawing the solution for the first time, set up pre-labeled smaller containers (e.g. Eppendorf tubes) and then aliquot the amount you expect to use for each experiment. That way potentially sensitive compounds are only thawed twice.
References:
1: Kozikowski, B. A., Burt, T. M., Tirey, D. A., Williams, L. E., Kuzmak, B. R., Stanton, D. T., ... &amp; Nelson, S. L. (2003). The effect of freeze/thaw cycles on the stability of compounds in DMSO. Journal of biomolecular screening, 8(2), 210-215.
2: US Food and Drug Administration. (2008). Guidance for Industry. Drug Stability Guidelines.