General reluctance:
* Hesitant
* Unwilling
* Disinclined
* Averse
* Loath
* Leery
Reluctance due to fear or doubt:
* Apprehensive
* Wary
* Dubious
Reluctance due to unwillingness to exert effort:
* Indolent
* Lazy
* Unenthusiastic
Reluctance due to stubbornness or resistance:
* Resistant
* Defiant
* Obstinate
Formal or literary synonyms:
* Reluctant
* Unwilling
* Disinclined
* Averse
The best synonym will depend on the specific context of your sentence.