Confused

Hi ladies! So I bought a digital basal thermometer about a week ago, and I read the instructions (because I am quite new to this) and long story short it says it's supposed to beep after approximately 10 seconds after my temperature is reached, it hasn't beeped once since I've gotten it I have had it in my mouth at the max over three minutes cause I thought well maybe it's slow. And still nothing. Do you think I should try to get a refund or is this normal? Any information will help. Thanks! :)