"Touch sensitive" keyboards don't actually detect how much pressure you're applying to the key-- unless they have "after-touch," which is used for something else (usually modulating the sound while you're holding down a key) and has nothing to do with determining how loud to play the note.
Instead, "touch sensitive" keyboards detect velocity, or how quickly you strike a key-- which usually corresponds more or less to the amount of force behind the keystroke, since playing a key "harder" or more forcefully usually results in striking it more quickly.
The way this works is that there are sensors in the keybed which detect when a key has gotten to a certain point while being depressed.
A keyboard that is not velocity sensitive has only one sensor per key, so they can detect only whether a key is depressed or not, hence the key always produces a note having a particular volume regardless of how forcefully/quickly you struck it.
Most(?) keyboards that are velocity sensitive have two sensors per key, one that detects when the key is only slightly depressed (or almost all of the way up), and another that detects when the key is fully(?) depressed. The keyboard compares the times when each sensor is triggered to determine how long it took for the key to go from one point to the other, then translates that time difference into a velocity and a corresponding volume.
Some keyboards-- especially digital pianos-- may have three sensors per key, with the additional sensor being partway between the other two. This lets the keyboard better detect when you strike a key, then quickly strike the key again without giving it time to return to its fully raised position. In other words, the first keystroke triggers all three sensors, but the second keystroke triggers only the lower two sensors because the key didn't have time to return to a point above the first sensor, if you see what I mean.
The PSR-E453 has only two sensors in the keybed, so if you repeat a keystroke before the key has had time to return to a point above the first sensor then the second keystroke won't trigger both sensors, hence the keyboard probably won't generate a sound for the second keystroke. Unfortunately, this is a common problem with keyboards that have unweighted synth-style keys, and even with keyboards that have semi-weighted piano-style keys if they have only two sensors per key.