This can be due to several factors. First, your battery bank may not need it. If your battery bank is full or nearly full, the controller must limit PV power to prevent overcharging.
Second, standard test conditions (STC) don’t always match actual environmental conditions at the install site. Thermal effects, atmospheric conditions, tilt, azimuth, and irradiance change PV performance. Check if your module manufacturer lists “NOCT” data on the datasheet. For many installations, NOCT data is closer to actual performance than STC data.
Also, your PV array and controller may be mismatched. If you have a PWM controller, the nominal voltage of your panel should match the nominal voltage of your battery bank. If it’s higher, the PWM controller is essentially “throwing away” the extra voltage it cannot use. For example, if you have a 12V battery bank, choose a 36 cell module. The Vmp will typically be 17 to 18V.
Other causes of power losses include loose wiring connections, module mismatch, dirty module, and orientation of the module to the sun.