So hanging back some, when coming into & through impact there's still too much weight on the trail side & subconsciously you're trying to help the ball up some, so there's an intention in there to swing upwards through impact.
You'll get away with it some if the balls on a tee, but not if it's on the ground.
With the weight on the trail leg sometimes if the hands/wrists don't bend upwards enough, flip, the club hits the ground first, if the hands/wrists flip in time you can get some contact but not full proper contact so that's why the 5i doesn't go as far as it should.
Sometimes the timing of this hand/wrist flip means the leading edge alone contacts the ball so depending where this happens in relation to the balls equator will mean either thins or tops.
The reason this happens, is with the weight hanging back it means with the 5i iron the lowest point of the swings arc is before the clubhead gets to the ball, simply because you're not really trusting the loft so hanging back on the trail side with the lift up of the body flick with the hands 'help up in the air' motion.
This means that with low point of the swings arc being before the clubhead gets to the ball, it depends then on the timing of the 'flick-up' hand/wrist movement as to where the ball or ground is struck first, so sometimes contact will be fat, sometimes with more of a earlier 'flick' you'll hit it thin or top it, but it's all part of the same kind of motion, just how & where it's timed as to the outcome at impact.
With any iron we need to swing down & through to get the ball 'up', so the lowest point of the swings arc needs to be target side of the ball, so ball first ground (divot) second.
Given the friendly looking lofted 7i you're feeling better able to swing down & through as you can see more loft at address, so with that club your low point is target side of the ball as it should be.
Just need the same intention of swinging down & through with the 5i as you have with the 7i, & trust the loft on the club to get it 'up'. It will do, you just need to picture the low point of the swings arc, is target side of the ball.
To then achieve that in the swing motion from the top you need to make sure that although your head upper body stays behind the ball, your weight at start of transition/downswing first needs to shift to be on the lead foot lead leg. then as long as you also clear the lead hip, you just swing downwards & through to make sure that lowest point is target side of the ball, so ball first, divot second.
From the top don't think 'helping up' & hang back, weight on the trail side, you need to think & do the opposite, weight left, swing downwards.
