Minecraft PE nether portal

Please post a screenshot so we can understand your issue better. There are a few possibilities, mentioned by Sum1.

1. You are not lighting the portal with flint and steel.

2. You are leaving a gap in the portal or have an extra misplaced block. Crying Obsidian will not work for nether portals.

3. The Nether is disabled in your world.

4. There is a flowing liquid obstructing it.

5. I believe in older Minecraft versions (I'm going off my Console experience), you could not make a Nether portal that strayed from the standard.

In general, we'll be able to assist better if you give us what version you are on, and show us a screenshot.