Size may be your problem.
Is the drive HDD (mechanical) or SSD ?
If HDD then:
2.5" hard drives normally come in one of three thicknesses:
* 12.5 mm - these are the Super-Phat drives that probably only larger laptops (DTRs and workstations) can fit. They usually have three platters instead of two, enabling higher capacity.
* 9.5 mm - this is the most common size and should fit most laptops without problems.
* 7 mm - these slim drives are used in thinner laptops (like ultraportables). Most '''SSD'''s are also in this size, and usually include a plastic bumper to increase the thickness to 9.5 mm in case the HDD mounting system requires it.