I'm trying to add text to an .ai file. The text shows up just fine in inkscape but when I view the image in an other program some of it disappears and some change in font size. I'm new to this program and have no idea why this is happening.

You should avoid using Flowed Text. Flowed Text is created when you click and drag out a box with the Text tool, before you start typing. To make regular text, just click once and start typing. You can convert to regular text after it's all typed. Text menu > Convert to Text.

The best way to make sure it stays the same would be to convert all the text to path, which is Path menu > Object to Path.

That solved my problem, thank you very much.