Inlines problem under admin: on production server I'm unable to add more lines in the inline section

[Victor]

In admin.py I have
.......................................
class MovimentomagInline(admin.TabularInline):
model=Movimentomag
extra=3
save_on_top=True

class MovimentoOperazioneOption(admin.ModelAdmin):
list_display = ('segno', 'data_movimento', 'paziente','operatore')
fields=(('segno','data_movimento'),('paziente','operatore'))
inlines=[MovimentomagInline,]
save_on_top=True
order_by=['-data_movimento',]

admin.site.register(MovimentoOperazione,MovimentoOperazioneOption)


Now it happens that - under admin - when I use "python manage.py runserver" in the developping context I can see in the inline section the "Add another ..." with the plus sign and I can add more lines in the inline section itself.
Submitting the same django procedure by means of Apache 2 I see the lines specified in the extra option but there's no longer the "Add another ..." with the plus sign, therefore I cannot add more lines in the inline section.

How can I fix that?

Thanks
Vittorio