I've only been able to summon about 10 times so far, but all but one or two of the times, I've summoned Ramuh. The cutscene is cool, but I have 3 other summons, so why haven't I called on them? Is who gets summoned randomized or are there factors that dictate who will get summoned and if so, what are they?
You're right, there are certain conditions, which is often based on environment, depending on which Astral it is. Here's an excerpt from a guide:
Titan: Chance increases with number of fallen allies. Can only be summoned in wide, open locations or flat terrains.
Ramuh: Chance increases with amount of time spent in battle. Can be summoned anywhere.
Leviathan: Can be summoned when Noctis is in trouble (during Danger status). Can only be summoned near bodies of water such as lakes or near the coast.
Shiva: Chance scales as the chances to summon all other Astrals increases. Can be summoned anywhere, given sufficient space.
Bahamut & Ifrit: Can only be summoned in the late game automatically.