encoder readSpeed() output 0 or 6.2500 only

Im working in making a small model to control DC motor using LM393 IR encoder. I'm using arduino as data acquisition and infrareed to get the rpm.

when i try to get rpm by rpm=readSpeed() it gives me zero or 6.2500 only why ?

here is the code

clear;

clc;

a = arduino('com3','uno','Libraries','rotaryEncoder');

Encoder=rotaryEncoder(a,'D2','D3',120);

PotPin = configurePin(a,'A0');

HBA1 = configurePin(a,'D10');

resetCount(Encoder);

while true

voltage= readVoltage(a,'A0');

writePWMVoltage(a,'D10',voltage);

rpm=readSpeed(Encoder);

pause(0.1);

disp('rpm = ');

disp(rpm);

end