I think if they wanted to buy the domain they would have by now. I agree with
@bojan1 you are in a situation where you lose if you contact them and lose if you don't. I also think that as
@korganian said they likely have little interest in buying it and are using the partner as an excuse. The way I handle something like this is - contact them again and be open and upfront with them.
I have this name that is helpful for someone in your industry. I want to sell it. You said you have interest. I respect that and wanted to give you the chance to have the right of refusal since you indicated interest. Let me know what your intentions are please because if you are not interested I want to go to your competitors ( I would include a couple actual names from your research here) and make them the same offer as I feel the domain would be a benefit to them and want to find a buyer that benefits from the domain. Please let me know what you think as I want to continue to try and sell this domain later this week, but want to give you the first chance since you indicated interest.
That creates urgency without desperation and also reminds them what the domain could do to help the competition.
A couple caveats. I don't know the domain but I am assuming you are selling a generic industry domain and not a specific company domain or a domain that only helps one party. If so I would not do this again for many reasons.
I would steer clear of buying a .co and spend time finding .coms for outbound it is much harder to outbound a non .com. That is a personal suggestion. Yes you can sell many tlds to end users every day. I am just saying .com is easier to sell in most cases. As
@MapleDots points out a .co is a hard sell to someone who already has an existing domain name they built their business on.
Some suggestions provided above to me sound deceitful. I would never suggest being anything but completely honest in your interactions with anyone. You only have one reputation and you should value that more than all else. Once lost it is very hard to regain. Not saying you would do that just making a general point.