[tbontob]

Hmmmm....

I forget how it plays out in a solo game vs the AI.

In a simultaneous game, it takes three turns:
1) The offer
2) The acceptance
3) Execution of the trade.

Assuming enough time has elapsed, you may want to check that each of the above was performed correctly.

For example, the offer may have been made for the wrong planet (unlikely as it sounds).

Or it was never accepted.