I use a light wallet to make transactions in IOTA (Android Wallet + Windows wallet in Light Mode). When I make a transaction, I always need to reattach my transaction to the system to be confirmed (I tried a lot of full node addresses, but i have the same behaviour). I know that the Trinity wallet will do that automatically, but why? it doesn't make sense to me. Why do we have to do that ? If the number of transactions was higher, should we do that too?
According to How does reattaching a transaction increase the chance for confirmation? two main reasons can be identified:
Your computer takes a lot of time for the POW, which results in an attach position not in front of the tangle. Thus other tx will not pickup your tx. (see also Does the tip selection depend on how quickly the proof of work is done? and Why does the tip-selection prefer new tips?)
Two tips may have been chosen that now belong to an invalid part of the tangle. This can be caused by your tx A confirming a tx B which was also reattached, but the reattached B' was picked up by the COO and confirmed. Thus your tx A attached to B is confirming the double-spend of B'.
As I understand you should do this to increase the chance of the validation, because you have more than one transaction. This transaction reference to other transaction, but is still a new transaction. Another issue are invalid transactions. If your transaction get confirmed by other, invalid transactions, your transaction will not confirmed anymore, because your transaction is within a parasite graph. It can then take much time until your transaction will confirmed.
This is what I interpreted currently. But I don't know if I'm correct.
Here is a great article : http://untangled.world/iota-consensus/
It explains a lot of things !