[color=#333333; font-family: 'helvetica neue']Hi,  It might be that another bridge has to be built in front of the well or that some constriuction has to happen on the well itself to get the symbol.
[color=#333333; font-family: 'helvetica neue']Also, it might be about the spider symbol.  If I remember, build a bridge to the web, drop on the web and then find the bugs (there's info on this in tips) then drop back on spider to get symbol. Meanwhile there is a symbol under the teleporter when it's uncovered.
[color=#333333; font-family: 'helvetica neue']By cave - do you mean the cave with the hermit or the stone face.  The stone face just needs to be uncovered and then get the crystals, drop on it and it coughs up a symbol.  
[color=#333333; font-family: 'helvetica neue']If hermit, you have to have the right levels and then drop on and talk to him and the build the statue to his left which gives you a symbol, the blueprints and the last gem for the teleporter.  This may not be inexact order and I'm assuming you have the levels already if you're so close to the end.  If not - check back in :-))  Good Luck!   
[color=#333333; font-family: 'helvetica neue']P.S.  there's a list of where all of the symbols are located below in this forum from mrspockfan[color=#333333; font-family: 'helvetica neue']