Hello ... have you tried stirring it and then standing it in a dish of tap-hot water and see if it finishes culturing once mixed through. You're using a starter and it may not be taking up the milk properly - stirring it may help, but warmth when you do will cause it to thicken and start to curd if you leave it that long.
Cultures are sensitive to even minor temp. changes - my kefir grains go into almost hibernation mode as the weather cools off and then into hyperdrive as it warms up - do you always keep your culture in the same place, is that place subject to temp. changes? If so - try moving it.
Also, you may be over-feeding the starter culture using raw milk straight up. Try leaving your milk in a glass jar overnight on the bench to 'age' and then using it as a starter milk ... you might find that the starter takes to it better than it does absolutely fresh.
When you get a suitable batch - reserve a small amount [about 1/20ths of the normal volume] to use in the next culture. Use at room temp, mix your new starter into it and then blend it into the new milk. It should help the new starter mix to take and reactivate.
Try reducing the volume of milk to one half of the recommended mix-ratio. If you're using raw milk, it may not be thickening because it is 'real milk, straight up' and the mix has been raised on a commercialized version that hasn't met raw milk before. It may be literally 'over-fed' - so try reducing your milk ratio by half and see it turns out a more consistent finish.
Hope that helps ...
Cheers,
Em