I believe the remote supplied with the Express+ sends its signals to the Roku by infrared (IR), which is direct line of sight only.
Be sure the front face of your Express is facing your viewing location.
Make sure there is nothing blocking the direct line between you and the Roku.
Check for interference with the IR signal -- some fluorescent lights may interfere, as can direct sunlight.
You can check if the IR signals are being sent in the first place by pointing your remote at a digital camera, even one build into a phone or tablet, and pressing some remote buttons. If the remote is sending signals you should see them flash on the camera view screen.
If you are still unable to resolve the issue you are experiencing, please send me a PM (private message) with your Roku account email address, and include the serial number/device ID on the Roku device along with a summary of the issue you are experiencing. I'll be able to assist you further from there.