1.) Not a DeveloperThe person you are saying this to is not developer. They don't know how to do it. PERIOD. At this point you are refusing to help them. This person may now have the opinion that your community isn't helpful. Don't ever for any reason assume someone is a developer, assume they are not. I would wager only 1 person in 10,000 is a developer.
I was once told that I should be able to fix a bug in git (msysgit technically) because git is a developer tool. At that time I couldn't code at all but was using git to store homework or something...
2.) Novice DeveloperSome how you know the person can code through some method other than assumption. They aren't as smart as you are, there code is so amateurish that it wouldn't be worthy of inclusion, even if they could figure out how to fix the problem. They can't get that far, they've no idea where to begin. You're probably just frustrating them instead of helping the problem. All people that you know are developers fall into this category (unless they fall into the next), either be willing to help this person patch your code or just give them the real reason (e.g. I've no interest in this problem) for not helping. These people know they could be helping, they may even want to be helping, but you telling them that they could be is not helping. They need guidance, not an invitation.
3.) Doesn't CareSo the person doesn't fall into any of the 2 above categories. Guess what? they don't care about it enough. They don't need you to tell them "patches welcome", they know what open source is. They know they could send you a patch, and that you might accept it. They know all that, but there's some reason they don't want to. You aren't telling them anything they don't know. Again just tell them the truth of why you don't want to work on it. Maybe ask them why they don't. Maybe it's a silly little barrier that you can help with. But only bring that up if you know they are competent enough to do it.
Just StopJust stop, after years of hearing this when I couldn't do it. All I hear is "fuck you I don't want to take the time to help you". Why would I want to help if that's what I hear? Don't tell me it's not what you mean. Because I've had that said directly.
Stop acting as though everyone can or should write you a patch. Most of you don't agree that you have a responsibility to the free software you create. No one else has a responsibility to contribute to the software. They aren't leeching if they don't give back, you gave it away. Don't ask them to, don't tell them to. They don't want to hear it anymore than you want to fix their problem. Definitely do NOT "volunteer them" to get something done, that's just plain rude.
Pretty much everyone you've ever told this too already knew that patches were welcome, so there's some other reason they weren't writing one.
Exception: If someone asks you if you mind if they work on your code. You can of course answer them "patches welcome" this is because they asked, as opposed to you volunteering that info.