Custom Validation Class not recognized from an external assembly

I have an assembly that has a validation namespace that stores all my validation classes (derived from BaseValidator). If I have that assembly in my bin directory, any aspx page does not recognize when I register with something like
<% @Register Namespace="MyProject.PageValidations" tagprefix="customvalidation"/>
<customvalidations:validator1 ......./> ---- Gives cannot compile
However, if I take the same class and put it in App_Code directory, works like a charm??

Any ideass???
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