At night time when you go to bed fall asleep with your hands on your stomach where the baby is and when you are just laying down and relaxing you probably will feel him or her moving. Also during the day just make sure you are relaxing and concentrating on the movement.
Im 21 weeks too and when I went to the docs babys it took him a while to locate where baby was...he eventually got a faint heart beat near my belly button..i prefer getting to hear babys heart beat at my ultrasound appointments..have you gotten to hear it while doing ultrasounds? Dont fret its there somewhere! But yeah ultrasound id rather go by that think my lil monkey was just being difficult to locate hehe