If using an in-house procedure, try a
Calcium assay kit suitable for your needs to investigate if the procedure or the material is the cause.
Unusable fluorescent signal indicates that the Kd value (dissociation constant) for the chosen fluorophore isn't suitable. Fluo-4 has a Kd value of 345 nM. Use an alternative such as
Calbryte™ 520 AM, with a Kd value of 1200 nM, or
Fluo-8FF™ AM, with a Kd value of 10,000 nM.