Building 3D printer: LM8UU bearing can't fit into
Bearing on a shaft
An 8 mm (ID, Internal Diameter) bearing should always fit on an 8 mm (OD, Outer Diameter) shaft. If it does not fit, one of the 2 is of a different size. Please use a caliper to determine the size of both.
Bearing in a housing
If it is the outer size of the bearing, i.e. fitting in a printed carriage or a bearing housing (e.g. SCxxUU), you should be aware of the different sizes of polymer bearings. E.g. there are RJMP-01-xx and RJ4JP-01-xx types of polymer bearings (xx denotes the shaft diameter, e.g. 08 for 8 mm shafts).
For RJMP-01-xx bearings (not compatible with LMxxUU bearings):

// Parameters for RJMP-01-xx (shaft size xx, all dimensions in [mm])
// Designation  d1  d2  B   B1      s   dn
//-------------------------------------------
// RJMP-01-08   8   16  25  16.2    1.1  15.2

For RJ4JP-01-xx bearings (compatible with LMxxUU bearings):

// Parameters for RJ4JP-01-xx (shaft size xx, all dimensions in [mm])
// Designation  d1  d2  B   B1      s   dn
//-------------------------------------------
// RJ4JP-01-08  8   15  24  17.5    1.1  14.3

d1 is the inner diameter, d2 the outer diameter and B the length of the bearing
The RJ4JP-01-08 bearings are drop-in replacements for the LM8UU linear bearings. Note that the RJMP-01-08 have a larger outer diameter and do not fit into the regular bearing housings. The increased thickness gives the bearings more rigidity.