Solutions for a bland vegetable curry
Here are some suggestions-

I usually use freshly prepared spices. By this what I mean is I buy from the market not the powder but the whole spice. For example I buy green chillies from market. I dry them under sun, and then grind them for a fresh chilly powder. Also I use two types of chilli powers Kashmiri, for giving red color but it is not spicy and the spicy chilli powder. I also buy coriander seeds from market and grind them fresh wherever I want to make indian curry. The aroma while grinding is so strong compared to the powder packet spices from market, and obviously this enhances the flavour.
Spices used initially, eg cardamom, cinnamon, cloves, I usually roast them and then grind them. This gives an amazing flavor to my curries. If you are not using these spices, I will recommend you to use them especially for Punjabi Graveys
As you mentioned, you use potatoes. And you fry them before adding to curries. That's good. You can also make holes in potato so that some flavors penetrate inside the potato and it doesn't taste bland.
Whenever I use onions, I chop them finely. This enhance the flavor to a great extent. As chopping them finely and then using them their surface area increases, increasing their flavor.
I hope you use fresh tomato puree.
Whenever you add spices like turmeric, chilli powder, then cook them little while and then you may add little water so that their flavours blend nicely.
You can try adding some curd or cream to your gravy.
However make sure to mix the curd well before adding.
You can try add some cashew pastes, or muskmelon seeds to especially punjabi gravy
Whenever buying vegetables from market, you should one with best flavors. Tomatoes should have sourness and curry leaves should have good flavor. You should smell them before buying.
After I have prepared my gravy I usually give it an oil/ghee tadka. This enhances flavor to a great great extent. By this what I mean is that I heat some oil/ghee in a small pan, add Asofoetida(optional), cumin seeds, curry leaves chilly powder mix well and as they start to sprinkle I quickly pour this is my gravy.
I hope these suggestions help.
Will add more if I remember one.