If you haven't got their balance ... you have no chance of getting ikkyo. If you truly have their balance ... they have no way to counter.
100% of ikkyo problems are a lack of taking balance, or, uke regaining balance (due to tori error or uke craft) shortly after.
Exactly what I was going to say, ikkyo isn't much to do with the arm, my teacher often demonstrates it holding your fingertips with a very gentle touch that's taken me almost ten years to begin to approximate.
You can't do that unless you've already got their balance.