Why is it not possible to scale certain parts?

Some typical reasons a part cannot be successfully scaled are:

Unable to scale a part up or down and the part contains a rebuild error.

Solution : Ctrl+Q the part with 'Verification on rebuild' enabled. If a feature failure is presented, this could well be the cause of the scale failure.

To enable 'Verification on rebuild' Go to Tools> Options > Performance

See below image for reference.

Now CTRL + Q the part, the issue will be resolved.

2. Unable to scale a part up or down and the part fails a geometry check.

If the part can be regenerated with 'Verification on rebuild' enabled and no failures are presented, perform a Evaluate > Check.

This may reveal geometry faults that the regeneration process does not detect. If this is the case, the part should be sent to your local support representative for investigation.

3. Unable to scale a part up:

The part may contain some tolerant geometry. As a part's size is changed during a scale operation (i.e. the part is actually scaled, the system does not simply use some formulae to feint a scaling operation), the tolerance of some geometry may fall outside of acceptable upper limits when a part is scaled up.

4. Unable to scale a part down:

The part may contain some short edges. As a part's size is changed during a scale operation (i.e.: the part is actually scaled, the system does not simply use some formulae to feint a scaling operation), some edges may become too short to be acceptable. Short edges can be detected using Evaluate > Check.