Asalamu Alaikum,
If you mean salah at masjid in congregation (gamaah) then its ok to pray alone if you were busy and couldn't free yourself at the time of gamaah.
If you mean that you miss the time limit for the prayer until the next prayer comes, then try to spare 5 minutes and find a free room to pray in or even just in the corner of any room, you can pray anywhere, on grass, on the beach (pebbles or sand) just anywhere where there is no najasa (uncleanliness such as animal excretion) and it is preferred to pray somewhere dry to avoid any doubt of najasa.
One of the advantages of islam, is that you can pray almost anywhere so long as where you pray is clean and it is easier for men as they don't have to pray in a hidden place like women.
I remember days when I was at University when I could spare 5 minutes in between lectures to pray Asr prayer, as the lectures end at Maghrib prayer. I used to find an empty hall where I would pray with my friends as the masjid was too far to reach before the next lecture started. We would put a jacket or our notebooks to pray on as the hall wasn't carpeted.
Between each prayer and the one after it are several hours and each prayer must be performed within its time limit or else it would be Qadaa. You musn't leave prayers and pray them as Qadaa on purpose; Qadaa prayers can be only if you miss a prayer without meaning to e.g.
- you slept and didn't hear the alarm clock
- someone is threatening your life
- you're unconscious
The importance of prayer reaches a degree that:
- if you can't find water for ablution, you perform Tayamum (involves hitting your hands on dust e.g rock or sand or even pillow twice once wiping you hands and then wiping your face).
- If you can't pray standing, due to illness, you pray sitting if you can't then lying down even to the point that if someone can't move (paralyzed) they can pray by moving their eyes.
Peace