Check the Configuration file to see what the portal requires. However, some modpacks may alter this behavior and change it to another item.

Are you using the correct activation item? Generally, the portal requires that you use a Diamond to activate the portal.

Note that as of 3.5.263, it isn't necessary to have Dirt or Grass blocks in the corners of the frame. Make sure that the frame completely surrounds the water, even the corners, and that the bottom of the portal is also Dirt or Grass.

Are there Dirt or Grass blocks? The portal can use anything that counts as a Dirt or Grass block.

If you're unsure if a plant will work, just stick to flowers in Vanilla. However if a certain block is not specified as being a Plant or Leaves block, even if it says so in the name, it will not count as a part of the portal.

Is the plant or flower a viable block? The portal isn't exactly picky with what you can use, such as flowers, vines, leaves, tall grass, etc.Note that as of 3.5.263, it is not necessary to have plants on the corners. Check that all corners of the frame have a flower, especially on corners. Are the flowers covering the whole frame? A common mistake is not surrounding the portal properly with plants or flowers.Make sure every block of Water is a Water source. Is the Water completely still? Just because the portal requires Water, doesn't mean that it will accept flowing Water.Is the portal too deep? The Twilight Forest portal requires that the depth for water be only 1 block.Here's some tips to help diagnose the problem. If you think you have successfully created the portal, but no lightning strikes, then chances are something might not be working properly. It is also possible to use Podzol, Coarse Dirt or Mycelium for the Portal, as long as it is a dirt block and has plants.

The forest mod menu