From the CSPro help page on checkboxes:
The length of the field must be a multiple of the maximum length of a response. For example, if each response code is one character and you have five responses, then a field of length five would allow for the selection of all responses. A field of length two would allow for only two responses (e.g., "pick the top two...").
This means that if the response length is one digit then you need to make sure that your variable is defined in the dictionary as type alpha length 20. If you are using two digit responses then your variable in the dictionary must be alpha length 40 (20 responses times length 2). Also since the responses are alphanumeric you need to make sure that all the values defined in the value set for you variable are the same length. For example if you have two digit responses then values should be "01", "02", "03"... rather than "1","2","3".