Well you have to understand the structure of those organizations. They might not know anything about IT and have someone they don't know offering to "fix it", they might think you are there to remove virus from their PCs. Lots of org wouldn't know the benefits of getting a proper IT management system before they see it. Sometimes you have to get started volunteering at that organization, see what they really need, and then propose to develop some IT projects that would help them fix real world problems that they have. Before they see what problem you could solve, you are just a volunteer that doesn't fit with their core activities and they wouldn't know what to do with you. Anyway, there are so many NGO out there that even if you get refused somewhere, find another one, there's no shortage.