Overworld Nether Portal Not Working

Started a new Modded world and in creating a Nether Portal it does nothing to spawn into the nether... I get the purple glow effects etc on screen when standing in the portal but never joins world however long I stand there. Tried on a prior save where I was in the Nether and it worked fine (took maybe 5 seconds). Not sure what I've done wrong in this new world. Am new to modded so wasn't sure if its a mod causing issue or if its a bug in 1.16.4? Is there any ideas on what to try? Let me know what you require to support this. Thanks