Excessively loose locks made from looser curl patterns can be crochet hooked tighter. Re-backcombing is not advised because it often creates permanent loops and bulges in the shafts of locks. Some people bidirectionally palm roll their locks while they are damp — but it admittedly takes an excessive amount of effort to see any lasting tightening effects from this method.
Tightly coiled and kinky hair types can be crochet hooked gently, but retwisting and/or putting them in a protective style is more common to manually compress the shaft of the locks.
However, manual tightening is not required to salvage loose locks. Time itself will tighten even the loosest of locks. Here is a photo composite showing a loose lock and that same lock tightened naturally three months and eight days later.