Text editor program stops working TASM 8086 assembly

Question

I ran into a problem while writing code for my task. What the program is supposed to do is this: at the end of the sentence, when i encounter a period (.) I am supposed to check if there is a space after the period and if the first letter of the next sentence is capital. So this is a part of my code which does the job. However when I run the program it doesn't do anything and the cursor just blinks without the program ever closing or displaying the edited sentences.

MOV cx, ax
    MOV si, offset readBuff           ;read buffer
    MOV di, offset writeBuff          ;write buffer
    MOV bl, 97                        ;lowercase 'a'
    MOV bh, 122                       ;lowercase 'z'
  work:

    MOV dl, [si]                      ;
    CMP dl, '.'                       ;check if symbol is a period
    JNE continue                      ;if no, then just print the symbol
    MOV dh, [si+1]                    ;if it is a period move next symbol to dh
    CMP dh, ' '                       ; check if it is a space
    JNE add_space                     ;if no, then go to add space
    MOV dh, [si+2]                    ;put the first letter of the sentence to dh
    cmp dh, bl                        ;compare it to 'a'
    jb  continue                      ;if below then print the symbol
    cmp dh, bh                        ;compare it to 'z'
    ja  continue                      ;if above then print the symbol
    sub byte ptr [si+2], 32           ;if lowercase, then make it uppercase

  continue:

    MOV [di], dl                      ;write symbol to writeBuffer
    INC si                            
    INC di
    LOOP work

  add_space:
    MOV [di], '.'                     ;first add a period
    INC di        
    MOV [di], ' '                     ;then a space; argument needs type override error
    INC di
    INC si
    CMP cx, 0
    JNE work                          ;go back to work