Pardon one more recommendation in a slew of others — ten others, at time of writing.
From reading your description of the situation, my intuition suggested this possible scenario to me — please bear with me, because I will explain by way of example:
When I chat on IRC, there is this one person who repeatedly sends me private queries requesting that I assist them with a little project of theirs.
This assistance they desire takes a shape of them outsourcing creative effort to me for devising scenarios for a certain fictional character.
When it began, things seemed simple enough. Over time, I learned that they were relying on me to make up for their own lack of creative exercise: they aren't presenting me with scenarios and asking for some a critique, but asking me to devise rationales and initial conditions.
It is likely that this person lacks certain interpersonal aptitudes. A form of Asperger's? Possibly.
I therefore attempt to tailor my interactions in a way which minimizes, as much as I can know, their emotional distress or trauma, while assisting them indirectly.
Usually I simply tell them some variant of “I'm not in the mood” or “not now”, but sometimes I ignore the query when I'm not actively engaged in another discussion elsewhere on that IRC server.
Okay, enough of that. Now, to your situation.
Other answers have addressed how you can phrase your requests so as to be clear that whether there is a certain deadline, or whether you would like confirmation that the message was received even if the recipient has not yet formulated an answer to your request.
These dangling messages of yours are not simply friendly chat, but are professional questions which involve other social protocol.
My recommendation is that you assess the root cause for the lapse of reply.
Other answers have mentioned various possibilities: that the person simply hasn't gotten your message yet, hasn't read it thoroughly, or has read it but does not know that you are awaiting a confirmation that they are thinking things over.
There is another which you should consider: how often are you, for lack of a more tactful phrase, pushy when someone says ‘no’?
When you are inviting another person into some manner of bargain with you, and they tell you that they aren't interested, or that they are but with some unpleasant stipulations or conditionals, are you yourself impolite or difficult with compromise?
If you are, then there is a chance that these other people are ignoring you simply because it is the easy way for them to mitigate the potential situation on their end.
If you think that this is a possibility, then it seems to me that the best approach would be for you to address that outright.
- Admit that you have been difficult or obstinate in the past.
- Inform them that you will endeavor to be more reasonable in the future.
- Offer them some incentive. This is not a trick for bargaining or an attempt to beguile them: this is a boon, offered with no strings attached, to make up for any past wrongs.
Now, for the formalities:
- Proceed with delivering your proposal.
- Mention any deadlines for when you require an acknowledgement of receipt, if necessary.
In the interest of being forthright, inform them why there is a deadline: This is not a tactic; it is only necessary if you would like to proffer your venture to another person and not to the both or more of them.
- Mention any deadlines for when action needs to be taken. This is when an agreement to proceed is required.
Finally — as other answers have put forth, — you need to consider whether the medium of “instant messages” really is the best for what you want.
When people see an IM, they usually associate that with brevity or with informal messaging. Of course, you can have IM serving in lieu of paging — e.g.
Where are you? Come up to the conference room ASAP
But, that's probably not your relationship with these people who seem to be ignoring your messages.