Lots of the movies suffer from "the Enterprise is the only ship in range".
Back in the 1980s I remember reading a review of III that made some very good points about the plot. For example why wouldn't the Federation authorise a ship to go and get Spock's body. And why,if Genesis is so sensitive, would they not send more ships to guard it. But looking past that, why do they even need his body? McCoy has his katra so why not just take McCoy to Vulcan? They don't know that Spock's body is alive at that point.
There's a line in IV that goes something like "the president said the transmissions were directed at the oceans" and then they deduce they were directed at humpback whales. But as far as I remember the president doesn't actually say the transmissions were directed at the oceans! (I guess a line was cut.)
Generations is just a mess, but I will sum up my thoughts with "Why doesn't Soran just fly a ship into the nexus?" It worked for him before. And i remember the Nitpickers guide made a very good point that even if the star blows up the MASS of the star is still centered on the same place it was in, therefore the nexus wouldn't change course.