Well the reason you are failing is that the spinner only awards a small amount of HP when completed so if the song is nothing but a single spinner you are probably not gaining enough HP to pass. Unlike all the oher gamemodes, Taiko HP starts at 0 and fills as you hit notes correctly and drains some of you miss. If you finish with 50%+ HP you pass.
As far as the spinners themselves they are completed by alternating don and kat. There is nothing negative with completing them early so just spam them finished as quick as possible. They only count as bonus points same as sliders so they don’t cause a miss or loss of ACC if you don’t complete them. So I wouldn’t worry about them too much.