Why was it difficult for American ground forces to clear areas of guerrillas?

Before the invention of hi-tech gimmicks, the military had to see what it had to kill. Guerrillas couldn't be seen...so their cover was destroyed...via Agent Orange. After phasing out AO, Rome Plows were used. But then the Tet Offensive of '68 solved the allies problems; the VC surfaced and was for all practical purposes destroyed. They still existed; but they were hardly an effective force. The NVA took their place, and they were a whole new force to be reckoned with.