/diy/ - Do It Yourself

On my 3d printer I have this switch which gets pressed by fillament to signal to the printer it hasn't ran out, but sometimes i get a false positive that the fillament ran out even though it did not and when i checked with a multimeter, the switch was pressed and and closed, so how in the fuck did PI get an open reading?
the reading is done on gpio on a raspberry pi, which pulls an input pin high, and the endstop keeps it pulled to ground in low state and it filament runs out it opens and the the raspberry reads the pin as high (with nothing to pull it low)and stops printing
so with a simple system like that how in the fuck am i getting false positives?