Wondering about a spotted salamander population

In late March I found a pool that had wood frog and spotted salamander eggs. Rafts of r. sylvatica eggs were already established, and I think I had just missed the a. maculatum breeding: the bottom was festooned with spermatophore and there were several dozen fresh egg masses. This is a fairly small pool, about 10 feet across, max 18" depth. It is quite remote, very difficult and ugly to access, and I like to think I'm the only one who pays any attention to it. Since the first visit, I've been returning weekly to monitor the populations. For the first month my dip net was bringing up larvae at a pretty consistent (approx) ratio of 1 a. maculatum to 10 r. sylvatica. For the last 6 weeks I have brought up zero a. maculatum. Today, zero again. Plenty of sylvatica. I really want this population of a. maculatum to flourish.
Several possibilities I have thought of:
*The a. maculatum have morphed. But a parallel group of 6 from that brood in a replica pool at my house has not yet morphed, and I think mine have better conditions for development.
*The a. maculatum larvae are better at eluding capture, maybe as older larvae they are inhabiting a different strata of the pool and my net is missing them.
*I have read that some populations in the northeast are dying out because of acid precipitation. In that case, older adults from the 70's or 80's may keep returning to the pool for their annual mating convention, but no pups are returning to the woods. If there is a threshold PH for a. maculatum larvae development of which someone can inform me, I will check that out.
*One dip several weeks ago brought up a very fat red spotted eastern newt. Predation? I was very tempted to do a dissection but didn't.
Any thoughts would be appreciated.

I would look for red efts on rainy days and see if you find any small ones. They are very easy to find walking around with their bright red color. That way you would be able to tell if the noto's are morphing. If they are, I would expect that the macs and opacums would be also. You could always take a Ph kit with you and test the water. I would expect anything under 6.5 would be fatal. I would think that larvae numbers would be dropping even without acidity problems.
Birds, snakes, frogs, aquatic insects, and the larvae themselves are all eating the larvae. The larvae may also be hideing in the leaves and mud. I recently found a noto larvae and a pickeral frog tadpole in some pond mud I had scooped up and brought home. Im sure the macs and opacums are there just not in the numbers you saw early on.

Ambystoma larvae are good at hugging the bottom when they get larger - it's hard to net them out unless you really shovel that mud sometimes.

Thanks for those answers, guys.