I agree with Tony, noise is your likely problem. Here is a software debounce routine that will make sure noise spikes < approx. 24 microsecs will be ignored (assuming 12 MHz Osc.).

; This routine waits till 6700 cycles from the pulse generator have been encountered.
; Assumes P1.1 has Pulse generator input.
; Uses R5, R6, & R7.

WT6700: MOV R7,#67 ;67 TIMES THROUGH / 100 LOOP
WL1: MOV R6,#100 ;SET UP / 100 COUNTER
;Wait for stable '1'
WL2: MOV R5,#6 ;6*4 MICROSECS PER LOOP = 24 MICROSECS
WL3: JNB P1.1,WL2 ;IF '0' ENCOUNTERED RESTART LOOP
DJNZ R5,WL3 ;LOOP UNTIL 6 TIMES THROUGH LOOP
;Stable '1' was found, now wait for stable '0'
WL4: MOV R5,#6 ;6*4 MICROSECS PER LOOP = 24 MICROSECS
WL5: JB P1.1,WL4 ;IF '1' ENCOUNTERED RESTART LOOP
DJNZ R5,WL5 ;LOOP UNTIL 6 TIMES THROUGH LOOP
;Stable '0' was found, dec counter
DJNZ R6,WL2 ;DEC / 100 COUNTER, LOOP IF NOT 0
DJNZ R7,WL1 ;DEC / 67 COUNTER, LOOP IF NOT 0
RET