First of all, try a thicker steak. 1.5" is the thinnest I would cook using my method - I usually buy closer to 2". 1" is too thin for using this method. Buy a 2" thick steak next time and I guarantee you'll get better results.

Besides thickness, are you allowing the steak to come up to near room temp before searing? That's one of the biggest mistakes a lot of folks make.

The final rest before serving is important - it allows the juices to stay in the meat, instead of running out onto the plate when you cut into the steak, leaving the steak less juicy.

If none of this works for you, maybe your thermapen is slightly off - try pulling at 115 instead of 120.

Hope this helps!