Ok, bear with me if these seem overly simple, since you say you've
never done this before:
1. did you cut the tip off the spout?
2. after cutting off the tip, did you pierce the seal? (poke a nail or
long wire down into the spout, or some caulk guns have a wire attached
for this purpose - the seal is at the base of the spout)
3. Is it an old tube of caulk that's been used once already and now has
a clogged tip?