Here are some training examples that I deal with in class daily :ukes trying to spin out of shionage in turn I go into kotegaeshi second turning there body out of ikkyo transferring into sankyo or yonkyo and lastly trying to put on nikkyo and before I can apply it there tapping out and the instructor sees this and tells me not to transfer into another technique and he demonstrates on them they try to spin out he takes them down to the Matt but when I do I,m being to rough WTH.
Agree with the above posts. Beginners don't "try" to spin out of shihonage....it is an automatic response by uke to incorrect technique. It can happen even to very experienced aikidoka working with very new beginners. It is a common situation for shihonage.
I don't want to assume but it looks like your instructor at that particular day is rather new a teaching since beginners can spin out of his technique. To correct this mistake is very simple, you bend your knees to match uke's height (if smaller than you) or extend more (if taller than you). bottomline, you need to adjust so that you and uke are almost same height to prevent uke from spinning out. Sounds trivial but unless you know the answers, you'll just be stuck wondering what's wrong.
It's also not good practice to change to another technique if the original technique doesn't work but that's just my opinion. you need to work with your partner to understand why the technique is not working. It's very easy to get frustrated if the technique is not working and switch to another technique to compensate and regain face to you and your partner. You learn backwards or nothing this way, unless its henka waza practice.
If uke does try to intentionally spin out, read again the above. there's no way they'll be able to with correct technique.