- how old is the rifle
- round count
- was barrel cleaned / run in
- how are you measuring powder
It can be very frustrating but equally rewarding when you find the recipe that suits your rifle. I have found the perfect within 20 shots but some have taken ages with 100+
As has been said bullet choice is so critical as some will never group whatever you do. My normal best groups with any calibre have been with a "medium" load of powder. COL is very important too and altering this you will see the groups open / close as you will with powder changes. Keep to the same brass as this helps with consistency.
For something to group and then NOT too seems odd.
Good luck
Rapid 7 / 10-22 ATN 2 Stiller Predator .204 Pulsar N970 Tikka T3 semi custom ATN MARS thermal.204 Tikka T3 Semi.223 Wylde IOR Operator Lawton 6mmBR Kahles Gen 3 - Stiller 6.5 x 47 Bushnell BDX - Winchester Red Performance SX3 12g