Peggy- What brand of investment was it? Some investments are really
Platinum investment comes to mind.
We use Ransom and Randolph investment. Kerr Satin Cast is another
great one to use as well.
We usually wait for several minutes, unless it's rose gold, before
dunking in water and have no problem with the investment coming off.
A sonic and steamer helps.
Waiting a week before casting shouldn't have caused this.
My suspicion is that it's the kind of investment you used.
Have fun and make lots of jewelry.