MSP430G2553: Problem with MSP430G2553 ADC

I am using MSP-EXP430G2 Launchpad (with MSP430G2553 microcontroller) to take samples from a voltage divider. To be more specific, I have an FSR (force sensitive resistor) sensor in series with a 10 K-Ohm fixed resistor as my voltage divider. I am powering up the FSR sensor using the Vcc from my launchpad. The output of the voltage divider or the input of the ADC is variable and dependent on the amount of pressure exerted on the FSR sensor (more pressure on FSR sensor generates higher voltage on the voltage divider output). I set up my code such that my microcontroller is normally in low power mode 3 (LPM3) and only when my up-mode counter overflows, an interrupt service routine (ISR) is generated to take a single ADC sample using channel 0 (A0) of the ADC. The trouble I am having is the fact that the sampled value from my ADC channel stays only in the range of 0 to 80 depending on the amount of pressure I exert on the FSR. In other words, I should see a value of around 1023 (because of the 10 bit ADC) when I exert a lot of pressure on the sensor. However, the ADC output only shows 80. In addition, when I remove the ISR section of the code and move the code to take the ADC sample from my main function, the ADC works fine (I have a value of around 1000 for ADC output when I compress the FSR sensor really hard). I will appreciate any suggestions/help to solve this issue. 

#include <msp430g2553.h>
#define CONSEQ_0
volatile unsigned int ADC_value=0;
void FaultRoutine(void);

int main(void)
{
WDTCTL = WDTPW + WDTHOLD; // Stop WDT

if (CALBC1_1MHZ ==0xFF || CALDCO_1MHZ == 0xFF)
FaultRoutine(); // If calibration data is erased run FaultRoutine()

//Configure Clock
BCSCTL1 = CALBC1_1MHZ; // Set range DCOCTL = CALDCO_1MHZ;
DCOCTL = CALDCO_1MHZ; //Set DCO = 1MHz
BCSCTL2 = DIVS_0; //SMCLK = DCO = 1MHz
BCSCTL3 |= LFXT1S_0 + XCAP_3; // clock system setup to run with watch crystal at 32 KHz
IFG1 &= ~OFIFG; // Clear OSCFault flag

//Configure pins
P1DIR = 0xFF; // all pins in port 1 as output
P1OUT = 0; // assign zero (low) to all pins in port 1
P2DIR = 0xFF; // all pins in port 2 as output
P2OUT = 0; // assign zero (low) to all pins in port 2

//Configure Timer
CCTL0 = CCIE; // CCR0 interrupt enabled
CCR0 = 3277; //0.1s intervals between ISRs
TACTL = TASSEL_1 + MC_1; // Timer clock source ACLK, (MC_1 is up-mode)

while(1)
{
_bis_SR_register(LPM3_bits + GIE); // Enter LPM3 with interrupts
}

}

void FaultRoutine(void)
{
P1OUT = 0x01; // red LED on
while(1); // TRAP
}

//Timer A0 interrupt service routine
#if defined(__TI_COMPILER_VERSION__) || defined(__IAR_SYSTEMS_ICC__)
#pragma vector=TIMER0_A0_VECTOR
__interrupt void Timer_A (void)
#elif defined(__GNUC__)
void __attribute__ ((interrupt(TIMER0_A0_VECTOR))) Timer_A (void)
#else
#error Compiler not supported!
#endif
{
// FSR is powered up using Vcc
ADC10CTL1 = INCH_0+ADC10DIV_0+ + ADC10SSEL_1; // Channel A0, ADC10CLK/1, ACLK as ADC clock
ADC10CTL0 = SREF_0 + ADC10SHT_0 +REFON+ ADC10ON + ADC10IE; // Vcc & Vss as reference, Sample and hold for 4 Clock cycles, ADC on
_delay_cycles(5);
ADC10CTL0 |= ENC + ADC10SC;
__delay_cycles(13);
}

// ADC10 interrupt service routine
#pragma vector=ADC10_VECTOR
__interrupt void ADC10_ISR (void)
{
ADC10CTL0 &= ~ENC;
ADC10CTL0 &= ~(REFON+ADC10ON);
ADC_value = ADC10MEM;
CCR0 +=3277;
}