Hard to say without more info. Some possibilities:
Page 77 in the manual:
Support units with an asterisk cannot be assigned to another headquarters, combat unit, or a town, city or urban hex during the current turn.
German Divisions and Soviet Corps can directly attach up to three support units while numbered German regimental and Soviet Tank and Mech Corps brigade break down units (i.e. 3/129 regiment) can attach one support unit,...Note that Soviet Corps combat units, such as Rifle and Cavalry Corps, that were built up from divisions will have any attached support units re-assigned and be unable to attach any support units while the unit is broken down into divisions.
22.214.171.124. SU to CU Attachment Restrictions
There is no range limitation to the transfer of support unit attachments; however, combat units must be in supply in order to transfer support units. Changing support unit attachments may require the expenditure of admin points (12.2.3). A specific support unit attachment can only be transferred once per turn and will be marked with an asterisk in the combat unit detail window to denote that it cannot be transferred again that turn. Axis Allied support units cannot be attached to combat units of a different nationality. For example, Finnish support units cannot be attached to a German Panzer Division.
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