What is wrong with my buck regulator circuit using the RY8310 IC?
Your (ferrite) inductor does not seem to be suitable for this power application. Neither its inductance nor its rated current meet the required specs. Essentially you are saturating your inductor very fast.

According to your application, you would need an inductor with the following specs:

$$L_{min}&gt;\dfrac{18V\cdot(29V - 18V)}{29V\cdot0.3\cdot1.2A\cdot1.2MHz}\approx16\mu H$$

$$I_{sat}\approx 1.5\cdot 1.2A \cdot 1.3=2.34A$$

The 1.3 factor accounts for the ripple current and the 1.5 factor for safety margin. You could increase the current driving capability at size and price costs.