Hey, hmarie!
It's a little difficult to determine what the problem is (if any) without knowing what kind of Arrowhead plant you have. Not all Arrowheads are bushy, so there may be nothing wrong at all. If your philodendron is happy, this plant should be as well.
Legginess is *usually* a symptom of too little light with these plants. A west window should be sufficient, but maybe there is some other factor? Is there a tree outside, or a curtain in the way? I'm sure you already thought of these things, but I want to be sure.
What you could try is moving the plant to a brighter location or providing supplemental lighting. You risk bleaching or scorching, but then you would know that the plant has reached it's light limit and can adjust accordingly.
I'm growing a compact Arrowhead with no fertilizer but lots of light, and it is REALLY bushy.
I hope this is of some help!
