Why is my masa mushy?

I think you're probably either making the masa dough too wet to begin with or you're not steaming them long enough. They should steam until the tamale easily peels away from the husk. The wetter your masa dough is, the longer the tamales will need to steam.

You also may have put too much water to steam them...

I know this thread is way old but I did just find it ?It took me years to figure out that it was my meat filling that was too wet, making my tamales take forever to steam.

Something that will help is tearing the leave of the tamales and get tying it around the pot or put a bag on top of the tamales and put tree chili pepper on top of it.

Okay, so I got really frustrated when my tamales that I worked all day on wouldn't firm up. I tried a longer cook time ... like 4 hours longer ... and nothing. Finally, I put around eight in a covered baking dish in the oven at 350 degrees for an hour and they came out perfectly. Thank goodness. I thought I was going to have to throw them out.