No output from ekf_node when fusing visual odometry and IMU [closed]

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Hi

I tried to fuse poses from a visual odometry and IMU in ekf_node. The IMU measures absolute orientations (heading in east = 0). Also I'm sure that my Visual odometry algorithm is performing well although not very precise

But when I subscribe to /odometry/filtered topic, all poses are zero with their covariances growing fast

What are the potential reasons?

Should I use differential or relative mode? If yes, for which sensor?

Let me mention that I don't have covariances on published topics of IMU and Camera. I read somwhere that robot_localization can handle zero covariances by replacing them with small values. Anyway, could this be the reason for my problem?