126. Only at the Next Service

21:3 One can only say a compensatory prayer in the service that immediately follows. If one did not do so, he can no longer make up for missed prayers. For example, if one missed both shacharis and mincha, he can compensate for mincha in the maariv service that follows it. He cannot, however, make up the shacharis he missed since two prayer services have passed during which he did not pray at the proper time. (The same is true if he did say mincha but didn’t say his tashlumin at that time – MB 108:17.)

21:4 On a day on which musaf is said, if one missed shacharis, he can make it up at mincha even though musaf was said in-between. This is because musaf can be said all day long, even until nightfall, so at mincha it is not as if the time for two prayer services had passed. One may not say his tashlumin prayer at musaf since its text is too specific to act as a substitute for the missed shacharis. In any event, if the time for shacharis has not yet passed, one may still say the shacharis Shemoneh Esrei, even though he already davened musaf. (If one missed both shacharis and musaf, at mincha time he first says mincha, then musaf, then his tashlumin for shacharis – MB108:16.)