Characters like Captain Falcon, Marth, Samus, and various mid-tiers should try very hard to not stay in shield. It's quite a bad spot to be in for any characters due to the limitations of options. But these characters in particular should be heavily based around movement and evasion, rather than blocking- especially Marth, who has a rather poor shield, and can be baited to attempt a shield grab without much effort.
If you find yourself in a match where you're staying in shield a lot, try to look at how you're being pressured to go into shield and evaluate your options to respond to such a scenario. Things like increased dash dancing, short hop aerials to stuff approaches, or in Samus's case, a quick up-b, could alleviate some pressure.