Hi Hassan, one trivial reason for this could be the difference between the emission filters of the two microscopes. If one of them has a filter with broader band, it may let through some fluorescence which is blocked in the other. Differences in excitation intensity and detection sensitivity could be another factor. Do you have any other fluorescence in the sample?

I agree what Radek Machan have said. Maybe some microscopy parameters such as emission filters, laser power and sensitivity of two different microscopies should be taken into consideration.