Make sure you are NOT including the numbers when you type the mnemonic in... it should ONLY be the words:
Also, double check the spelling of the words... there are a few that a quite similar... I helped a fellow user a few weeks ago, one word in their mnemonic was written down wrong... you can get words like this:
So a missing letter or an accidentally added letter or changing of 1 letter can mess everything up!
So, if it isn't the numbers and you are typing 12 words, I would suggest that you look at the list of words here: https://github.com/bitcoin/bips/blob/master/bip-0039/english.txt
See if there are any words that are very similar to the ones you have written down (like kid/kit/kite) and maybe try different combinations