wvmarle:
OK, then there you have your answer.After 1.5 minutes your power bank switches off for lack of current (even active sensors will be drawing too little for it to detect), sensors lose power, and the Arduino reads zero distance (as the pulseIn times out and returns 0).
Just connect the sensors to the Arduino for power. They don't draw much, not worth messing around with external power supplies for this.
Also, next time use a real power supply. Not a power bank.
Yup, you're right. I measured the current drawn by each sensor and it's about 5mA each. I contacted Anker and they replied:
"Please note that the minimum current requirement (the lowest amount of power that a device needs to draw from the battery to make it not shut off) of this battery is 50 mA. If your device draws a power lower than 50 mA, then the battery will shut off. As far as I know, you can charge your device with 15mA and another device draws power higher than 50 mA like your phone from this battery at the same time (as there are 2 usb output ports) to stop the battery shuts off."
To confirm everything, I tested the sensors individually with a set of AAA batteries and they worked as intended. On the other hand, I plan on using the power bank for other things too so it'll draw a heavier current and thus won't shut down. Hope this helps anyone else who has a similar problem.
Thanks for the help all!