3 things . Hash rate, difficulty and possibly your fee.
Network hash rate? How does difficulty affect it? Why would the fee be a factor? I was depositing 2 equal amounts to a trade market and one was faster then the other one.
What coin are we talking about here? And how far apart were the transactions?
Hash rate and difficulty come into play because if the difficulty is very high but the hash rate is very low, it will take quite a while to get the number of confirmations on the network for someone to accept the transaction.
As far as fee is concerned if you send a coin with no fee or lower then normal, it will be lower in queue then those with higher fee.