A V shaped two wire arrangement so one will be visable on each tack. I've made dozens from coathangers and duct tape over the years with either yarn or cassette tape for indicators. Feathermate makes an excellent one that snaps on and off and comes with yarn and feathers for indicators.
JMHO, but I prefer the 1/4" cassette recording tape for really light air. Some folks go as far a using a single thread for light air, but my eyes loose it...LOL
Also a straight up one at the top of the mast works best for down wind. Again I jsut use a piece of coathanger taped to the mast with a length of recoding tape for he indicator.
The front spar mounted ones act funny going down wind with the wind coming off the sail. The on at the top of the mast is in free air.
It sounds like your mast is not turning correctly. I use a Davis spar mounted indicator. I noticed the same thing. Then I noticed my mast wasn't turning and my spar was rolling over on one tack. I have fixed the turning problem and have no more problem with the indicator. Wrap the mast with UMVH or glide tape and it will turn properly.