The slope of the shower base is not quite right. Water will drain correctly at the front of the shower, but the water at the back does not flow towards the front. Likely because the floor is not really straight. There might be a very slight bump in the middle that causes the water at the back to stay trapped.
I have no appetite for breaking everything and replacing everything.
Is there a solution for this problem? Maybe a type of silicon or something that could recreate more of a slope at the end?