If you know that your emission measurement is correct, then you need to improve your sample preparation method. The Er was not doped and integrated into your host correctly by the current experimental approaches.

You should first prepare your sample with the optimal or sub optimal respective concentration. of two dopants with the highest PL Emission according to the literature. Then you could improve or modify your experiments by PL and other techniques like XRD. Once you found an appropriate sample preparation method, you cam vary the one or two dopant(s) concentration. If you cannot find PL under some concentrations, which could be ascribed to ILS (infrared stimulated luminescence), such that one dopant acted as luminescence centre and another electron trapping centre.

Please refer to one of our publications from my PHD many years ago by the following link with available full text:

https://www.researchgate.net/publication/243278831_Study_on_relationship_of_luminescence_in_CaSEuSm_and_dopants_concentration