i have recently started breeding for the next season but been finding that new horses rating deosnt increase from 2 years old to 3 years old any one else noticed this?

Assume you mean after running them?

Game rating is of little consequence if your talking the next league.

i mean the potential rating bar doesn't increase from ages 2 to 3 anymore in my game

How many horses and seasons are we talking? It does not always change 2 to 3 or may increase towards the end of the 3yo season. Could be an issue could just be your stable, hard to say right now.

about 30 horses never noticed any not improve before

I'd agree that is unusual but may not be an issue. Save and back up your game cycle through a few more seasons without saving and see if it continues. If it does I would go onto bugs and report to mark.

I find that stats like cruising burst and the vertical speed bar are the main two that change when a horse turns 3. Cruise bar always improves but the vertical speed bar can decrease - oh and the vertical acceleration bar moves as well from memory.
Potential does improve with some horses as they age but only for a few (if that).
The stamina bar changes randomly with 2yos and I have seen it change in 3yos as well.

Best bet is to screenshot your 2yo stats at the start of the season (or take pics on your phone as I did) and compare them when they all turn 3.

PS I play the flat Australian circuit and stats may be different with a jumps game.

This sometimes happens with me. Most of my horses develop the most when 2 and 4. At 3 they just tend to fill their Stamina and Cruising Burst.

It depends on your breeding lines when a horses potential goes up.

My flat horses are early developers and potential always goes up from 2yo to 3yo and sometimes again at 4yo.

My jumpers are late developers and potentail doesn't go up from 2yo to 3yo. They do go up from 3yo to 4yo and again go up from 4yo to 5yo.