Suppose I assert a proposition p, and you assert p. Then we agree, don't we?
But suppose that propositions are tensed in the way presentists typically think, so that they change in truth value. Then it could happen that I assert p, you assert p, and yet we disagree. For instance, suppose we are watching a gymnast practice. I say: "He's only now got the hang of it!" A minute later you say: "He's only now got the hang of it!" We plainly disagree. Yet on the tensed proposition view, we asserted the same proposition. On an untensed proposition view, we asserted different propositions, say <He's only got the hang of it by t1> and <He's only got the hang of it by t2>, where "t1" and "t2" are names of times (this works best with a non-Millian view of names).
A necessary condition for me to be convinced by your assertion is that I come to believe what you asserted. But not so if propositions are tensed. You say: "She's scoring a goal." Reflecting on your reliability for a bit, I accept your assertion. But if propositions are tensed, what I come to believe is not the tensed proposition that she's scoring a goal, but the tensed proposition that she's scored a goal. It's not easy to get a tenable view of untensed propositions that gives the right answer here, but it's probably not impossible.