What can be the reason for converge and diverge of the difference in NIFTY future-future?

Their are 3 NIFTY future contracts open on every new expiry.

Future 1, future 2, future 3.

I know that difference in Nifty future 1 and spot converges on the date of expiry.

But what is the reason between convergence and divergence of difference between future 2- future 1, future 3- future 2 ?.

And How one can find out that this difference will converge or diverge in future?

On the day of expiry, Fut 1 and spot converges…but Fut 1 with 2 or 2 with 3 or 3 with 1 may not converge.