I believe strongly in God's providence. I consider every provision by Him to be sign that teaches. So, for example, as he provides me with my daily physical needs I see that as a sign of grace and it reminds me that I am dependent upon Him.
The bible also tells us that the very fact that evil is not allowed to run rampant and unrestrained is also a sign of God's Presence.
There is a resistance against evil and that is the goodness of God, there is a resistance against Darkness and that is the Light of the Glory of God in Christ Jesus
As far as why signs are not always the same there is no definitive answer but I believe it is because man can duplicate a formula, in the same way he can simulate a pattern and thereby recreate the similitude of a miracle but he cannot originate an authentic miracle
So there is one burning bush with a voice speaking out of the midst, there is one miracle of manna, there is one miracle of parting the Red Sea and one miracle of Jesus walking on water.
I think this is because God is a personal God and He gives signs and miracles specifically to and according to the need of a person and a people, for the specific need at that specific time.
This is why Jesus never healed the same way twice, because people are different and the miracle they have believed for is specifically for them.
The miracle of Salvation is the greatest miracle of all and every one who believes is Saved according to their faith. I believe that is because our interaction with Jesus is based on our need. Some would say that is a selfish Christianity that we should just Love the Lord because of who He is....and I agree that is true, but that happens eventually as we go along the Way, for as we go we grow, we begin to appreciate more and more how much He really loves us and what that really means, in turn we love Him
The bible says it like this1 John 4:10 In this is love, not that we loved God, but that He loved us and sent His Son to be the propitiation for our sins
Lastly I believe God does not want people's faith to depend upon signs but rather for signs to be in response to their faith. For a dependence upon signs makes one fickle with a tendency to waver and too easily mislead by lying signs and wonders
We see in the wilderness that no matter how much God showed the people no matter how many signs He gave them, those signs did not lead them to a place of rest and a Saving Faith in GodJohn 6:26 Jesus answered, “I tell you the truth, you are looking for me, not because you saw miraculous signs but because you ate the loaves and had your fill.
Jesus addressed this after he fed 5 thousand. They thought it was the work of a community activist, a social worker but from Jesus perspective He was giving them a sign.
So instead of pampering to their desire for more bread, more food by which He could've grown His followers into millions, He instead gave His hardest sermon ever and the bible says many departed and never returnedJohn 6:66 From this time many of his disciples turned back and no longer followed him.
Jesus did this because He only wanted true believers, those who could hear and receive the spiritual manna, because Jesus knew signs without faith are not enough for people who do not really believe!
Hebrews 11:6 And without faith it is impossible to please God, because anyone who comes to him must believe that he exists and that he rewards those who earnestly seek him.
A wicked and adulterous generation looks for a miraculous sign, but none will be given it except the sign of Jonah." Jesus then left them and went away.Matthew 16:4
Signs confirm our faith and should not be the other way around, in the kingdom of God that would be like the tail wagging the dog
My thoughts, hope it helps