New Here and Citi CLI denied

[WolfChild1]

Greetings. I am sort of new here - I've been a member for a while but never posted.  I am frustrated due to my scores not changing much and I am learning so much about why that is.   Thank you so much for providing this forum for everyone to work on their credit!  I have only two credit cards each Citi cards - at one time both had a 7000 CL but went down (CLD) to 500 CL in 2014.   Even though I had excellent payment history with Citi (late one 30 day late in 6 years) when I paid them both off in 2014 they SP my credit report and noticed I had a tax lien and repo in 2011/2012 (both paid, but were very new).  Anyway, I've not asked for a CL increase because I knew most likely I'd not qualify with the wreckage of my past on my credit reports.  

Today FICOs are currently TU: 700 EQ: 668 and EX: 638.  I think the reason why EX is lower is because they havent' removed/deleted an unpaid utility bill that hit my credit in Feb.  I didn't know about it until it was on my credit because I had moved three times since I had that particular account closed, so I paid it immediately.  TU and EQ kindly removed/deleted without even requesting, but EX is just showing "paid".  

Here is what I am doing after browsing the FICO forum.  I applied for Barclay Aviator card today since my TU is 700-740 (based on the info here thought that might be a safe bet) and I was also pre-approved for a Delta Gold AMEX so applied for that as well after checking out their site today and entering my info.  I only have one inquiry from on CR from 9/2017 when I was solicited for a refi on my car loan, which was stupid because it was my exisiting lender WFDS who HP (thanks Credit Karma!).  Additionally today requested a CL increase on both Citi cards for 2000 each online, but two hours later received an email denial letter but no explanation why. I had 1X30 day late in 2013, but otherwise pay them off each month sometimes several times pay them off.  I called Citi CSR to ask for reasons on the denial.. and CSR said she would try again for me.. and entered my information manually with a lower increase request of 1000, but have no confidence anything will change.

Any feedback would be most helpful. I have been stuck at between 640-700 for well over a year now and thinking it must be due to not having enough open credit cards!