The locking collar on the hose end is likely cracked or the locking tabs inside the wand/canister port are damaged. Inspect the gray plastic locking ring on the hose end. Rotate it to the unlock position and look for any hairline cracks, especially near the seam.
Also, check the plastic port on the canister and the wand. Look for broken or missing plastic tabs inside. Debris lodged in the port can also prevent a secure lock.
IF THAT DIDN'T WORK: If the locking ring is intact, the issue may be wear. Ensure you are pushing the hose end in fully until you feel it seat, then rotate the locking collar to the 'lock' position. You should hear and feel a distinct click.
HEADS UP: A hose that won't stay connected creates a major suction leak and makes the vacuum useless. Do not tape it. This is a parts failure. The hose assembly is a replaceable part. Contact Shark for the correct replacement hose for model NV356E.