When a target is currently ragdolled, such as by Pull, Crush, Melee, or another Warframe ability the Mag casts Magnetize on the target and consume energy, but the ability does not go into effect and must be recast. See below imgur video for example.
This bug is easily reproducible in the simulacrum and in-mission by casting Crush, then casting Magnetize. If the target is slowed by cold status, Gloom, or Nova's Molecular Prime, the window to cast Magnetize and fail is significantly increased.
https://imgur.com/a/Dq68kAI