A mourner who is having an operation, or travelling, or any other reason that makes it impossible for him to say Kaddish at a particular Tefila. Is there anything the mourner should do to make up for the missed Kaddish?
The minimum requirement for kaddish is to say it once at any one of the three minyonim. If a person cannot attend any of the three minyonim on a particular day, he cannot "make it up" on a different day. Chabad.org (shown below) recommends that one learn or give tzedakah or emphasize a mitzvah in honor of the deceased parent. This can help in the event that one has had to "miss" the opportunity to say kaddish at a minyan.
Note that the main requirement is a son for one of his parents.
Kaddish is not an all-or-nothing proposition. If for some reason a person misses a daily service, he continues saying kaddish in all subsequent services.
Ideally, kaddish is said three times a day. If it is not possible to attend synagogue for more than one daily service, once is the minimum requirement. If one is unable to make the commitment to say kaddish daily for eleven months, it is possible to pay someone in a synagogue to say it on your behalf.
Note that three times a day above means at each of the three services. It does not need to be said more than once each service.
What should be done, in such cases, is what the Kaddish itself seeks to do: enhance the "Merit of the Children." The mourners should read a portion of the Bible--a chapter from the Five Books of Moses or the Prophets--or, if he is able, study a mishnah or page from the Talmud. This is a constructive and entirely valid substitute for the Kaddish, when one finds it extremely difficult to attend one of the services.
Tradition recommends other ways to glorify a parent's teaching. Children should make a standard practice of contributing to charity in their parents' memory. Even more effective and more beautiful, mourners should strive to adopt one mitzvah, one special deed, which they will take to heart and practice regularly as a memorial tribute. This custom adds life to the influence of a parent who has passed on and builds a future life for those who survive.