modifying hc-sr04

i bought these transducers

what i notice is if my hand is really close to the sensors sometimes i can get a reading that i know is correct but then it right back to garbage. im wandering if it may not be enough power to drive them? when my hand is more than a few inches away i cant get anything.

Update: if i leave it somewhere under a hard surface i consistently get a good reading then a bad reading

example of readings

-6850
56
-6846
56
-6848
56
-6824
56
-6844
56
-6806
56
-6770
56