I think this is much more common than people think (at least this kind of stuff happens with me too). I think a good way to refresh your memory in such cases is to have a simple enough problem for techniques (it may be the first problem you ever solved using that technique, in contest or otherwise), or go through your old solved problems once again. I have yet to try these on a larger scale for all things I think I am forgetting, but it works well at the scale of a couple of techniques.

If you're forgetting things like tree dp, then perhaps it's more of a problem of not having had enough practice, or having a clear picture of the technique in mind (not talking about the understanding). Good luck!

it comes with time.