C language-if statement issue

I've created the following program to make a Caesar and Vigenere cipher. I've gotten the Caesar cipher to work correctly, however I cannot get the Vigenere to work properly.

What I want to happen is have my if statements "catch" all the various numbers with an interval of 5. However whenever I run the program my Viegenere cipher outputs the exact same input. I believe this is because I'm making a mistake with my if statements, but i'm not sure what it is.

The beginning of the Viegenere cipher is commented in the code as //Vigenere Cipher-- keyword is "apple"

#include <stdio.h>


int main(){
int i=0;
 //setting the individual slot number for the array-- later used in the while loop
char guy[100];
printf("Enter the plain text:");
fgets(guy,100,stdin); //takes user's input-- such as "abc" and puts it into its respective slot in the array guy[10] r-right?

while (guy[i] != '\0'){ //while loop that runs until it reaches the end of the string


    if ((guy[i]) >= 'A' && (guy[i]<= 'Z')){ //moves capital letter values up 1
        if (guy[i]=='Z'){
            guy[i]=guy[i]-25;
        }
        else {
        guy[i]=guy[i]+1; //makes the current "slot" number go up 1 value. Example: a = 97 + 1 -> b = 98
        }
        }
    if ((guy[i]) >= 'a' && (guy[i]) <= 'z'){// moves lower case letter values up 1
        if (guy[i]=='z'){
            guy[i]=guy[i]-25;
        }
        else{
        guy[i]=guy[i]+1;
        }
    }
    i++; //moves the array's interval up to the next "slot"

}
printf("Encrypted text is: %s\n",guy);

//Vigenere Cipher-- keyword is "apple"
//a = 1  value shift
//p = 16 value shift
//p = 16 value shift
//l = 17 value shift
//e = 5  value shift

printf("Enter the plain text: ");
fgets(guy,100,stdin);//takes user's input

while (guy[i] != '\0'){ //while loop that runs until it reaches the end of the string

    if (i%5==0 || i==0){ //checks to see which character it is in the string, for instance the numbers 0,5,10,15,20 should all be added by 1
    guy[i] = guy[i]+1;
    }

    if ((i-1)%5==0 || i==1){ //all numbers that are second in the key word 'apple', such as 1,6,11,16
        guy[i]=guy[i]+16;
    }
    if ((i-2)%5==0 || i==2){// all numbers that are third to the key word 'apple' , such as 2,7,12,17,22
        guy[i]=guy[i]+16;
    }
    if((i-3)%5==0 || i==3){// all numbers that are fourth to the key word 'apple', such as 3,8,13,18
        guy[i]=guy[i]+17;
    }
    if((i-4)%5==0 || i==4){// all numbers that are fifth in the key word 'apple', such as 4,9,14,19
        guy[i]=guy[i]+5;
    }
    else {
    i++;
    }
    }
printf("Encrypted text is: %s\n",guy);
}