Trying to accumulate wealth is like trying to prevent ice from melting.
You can heap on more and more ice to your stockpile. But you can't escape the fact that ice will start melting almost as soon as it is created.
If you are heaping more ice on top, it might look like you have reversed the trend; but remember that the melting is still happening - it is just less visible to you.
Inflation, taxes, downturns, speculative bubbles, regulation, bad investments, misfortune, depreciation, devaluation; any number of things can cause the melting - but melt it will.
This should not cause any alarm or concern. Neither should it prevent from trying to slow down the process through wise investments. It is just good to know that this will happen and to not be too alarmed by it. Anticipating an inevitable phenomena is much easier on the nervous system than being surprised by it.