Lower the Rate to Feel the Drive

If you're taking more strokes on the ergo but not getting much faster, I'd leave the rate alone for a bit. Have a look at the drive.

You can feel busy. You might be rushing towards the catch and cutting the finish short to get back there sooner. Plenty of movement, but you've given yourself less stroke to work with. Pulling harder won't necessarily fix that.

I'd slow the recovery down, giving yourself more time coming forward without making the drive softer. Feel for pressure on the handle through the drive, rather than a tug at the catch that disappears before you've finished pushing. A firm drive you can repeat. Not every stroke as hard as possible.

Keep your length, but don't reach further just to make the stroke longer if you lose control getting there. The lower rate gives you time to notice where the pressure fades, and if the output starts dropping, check whether you've shortened up or started rushing again before adding more effort.

There's nothing wrong with rowing at higher rates. You need to practise those too. When you bring the rate back up, see whether that pressure is still there. If you're only moving the handle backwards and forwards faster, come down a little bit until you can feel the drive again.