Encoder problem

[precision]

Hi all

I am using 900 ppr encoder for length measurements. i using 595 chip to show display and rs232 output for x,y plotting. when encoder rotate on 5 rpm, it is work. But when encoder run above 5 rpm, pulses are missing.
Where my codes are wrong ?
'---- chip 16F877 i/p 20 MHZ ---"
DEFINE OSC 12
INCLUDE "MODEDEFS.BAS"
ADCON1 = 7
INPUT PORTA.0
TRISA=0
TRISD = 0
TRISC = 0
TRISE =0
OUTPUT PORTD.2
OUTPUT PORTD.3
OUTPUT PORTD.4
OUTPUT PORTC.4

Symbol HC_latch = PORTE.0 ' pin 8
symbol HC_Data = PORTE.1 ' pin 9
SYMBOL HC_Clk = PORTE.2 ' pin 10


B1 VAR BYTE
B2 VAR BYTE
B3 VAR BYTE
B4 VAR BYTE
B5 VAR BYTE



DIGIT1 VAR BYTE
DIGIT2 VAR BYTE
DIGIT3 VAR BYTE
DIGIT4 VAR BYTE
DIGIT5 VAR BYTE
DIGIT6 VAR BYTE

MASK VAR BYTE

B1 = 0
B2 = 0
B3 = 0
B4 = 0
B5 = 0


E1 VAR BYTE
E2 VAR BYTE
E3 VAR BYTE
E4 VAR BYTE
E5 VAR BYTE

OPTION_REG = %00000000
On Interrupt Goto myint
INTCON = $90


IF PORTA.0 = 1 THEN NINE
LOOP: serOUT PORTC.6,T9600,[#b5,#b4,#b3,#b2,#b1,13]


'------------------------------------------
DIGIT1 = B1
LOOKUP DIGIT1,[$7E,$18,$6D,$3D,$1B,$37,$77,$1C,$7F,$3F],MASK
E1 = MASK
'------------------------------------------
DIGIT2 = B2
LOOKUP DIGIT2,[$7E,$18,$6D,$3D,$1B,$37,$77,$1C,$7F,$3F],MASK
E2 = MASK
'-------------------------------------------
DIGIT3 = B3
LOOKUP DIGIT3,[$7E,$18,$6D,$3D,$1B,$37,$77,$1C,$7F,$3F],MASK
E3 = MASK
'----------------------------------------
DIGIT4 = B4
LOOKUP DIGIT4,[$7E,$18,$6D,$3D,$1B,$37,$77,$1C,$7F,$3F],MASK
E4 = MASK
'---------------------------------------
DIGIT5 = B5
LOOKUP DIGIT5,[$7E,$18,$6D,$3D,$1B,$37,$77,$1C,$7F,$3F],MASK
E5 = MASK
'---------------------------------------

HC_Latch=0
shiftout HC_data, HC_Clk, LSBFIRST,[E1,E2,E3,E4,E5,$7E]
pauseus 1
HC_Latch=1

'---------------------------------------

GOTO LOOP


Disable
myint:

FF: B1 = B1 + 1
IF B1 > 9 THEN TWO
goto HH

TWO: B1=0
B2 = B2+1
IF B2 > 9 THEN THREE
goto HH

THREE: B1 = 0
B2 = 0
B3 = B3+1
IF B3 > 9 THEN FOUR
goto HH

FOUR: B1 = 0
B2 = 0
B3 = 0
B4 = B4+1
IF B4 > 9 THEN FIVE
goto HH

FIVE: B1 = 0
B2 = 0
B3 = 0
B4 = 0
B5 = B5+1
IF B5 > 9 THEN NINE
goto HH

NINE: B1 = 0
B2 = 0
B3 = 0
B4 = 0
B5 = 0
goto HH
HH:
serOUT PORTC.6,T9600,[#b5,#b4,#b3,#b2,#b1,13]

INTCON.1 = 0
resume
Enable