NGT question/troubleshooting

I work on a med-surg floor and was assigned a patient that had orders for an NGT to low-continuous suction for management of a SBO. The patient had a 16 Fr. salem sump inserted in the ED, with a lopez valve connected to the sump tubing and the remainder of the suction tubing was then connected the to wall-mounted, low-continuous suction. The patient's HOB was at 30 degrees, and the unit's wall-mount had the bottom of the suction canister just above the bedframe.

After shift change, I noticed that the patient's gastric contents in the suction canister were bubbling/foaming continuously within the canister, and at the site where the patient's tubing was connected to the lid. The canister had roughly 100mL in it. I checked the connections, and verified that the suction was functioning properly. Next, I checked the canister, lid was intact, ports were capped, and there were no leaks in the tubing. I noticed that if I lowered the canister lower than the patient's head, the tubing flowed freely with suction. However, when the canister was returned to the holder, the tubing continued backing up. I checked the blue pigtail to ensure it was functioning by holding my finger over it, there was slight suction noted.

Just looking to see what others would've done in this scenario. Thanks in advance!