The TS4 monitoring (either TS4-M or TS4-R-M) is a passive unit to allow most accurate measurement of the PV module it's installed on. As result, if the unit's input requirements aren't met, for example, due to very sever shading or broken diodes, module straps or others, the TS4-M unit may stop communicating. For the input specification of TS4-M please refer to the product datasheet.
Other Tigo products have active mechanisms that prevent them from suffering communication loss due to a voltage drop, for example. Such covers, like the TS4-O and TS4-R-O, provide you with a more robust communication, in addition to the greater energy harvest.