Unfortunately, generics can vary wildly in efficacy & quality. As @Aradina pointed out, sometimes the encapsulation is different (e.g. extended release coating vs. standard release), but also the form of the drug can differ (e.g. capsule, tablet, softgel, chewable, etc), chemical by-products from different manufacturing techniques may be present in different amounts, and different manufacturing processes can also yield different chiral enantiomer ratios in the end product.
The "same" drug from different manufacturers may vary in effectiveness / side-effects, and brand-name drugs aren't always the best formulations for most patients.