In general, the Fed Funds rate is below the repo rate. That is because of a few things:
With the introduction of interest on excess overnight reserves (IEOR) banks can park their money at the Fed and get paid for them. They have less of a need to move their money. This means that banks with idle reserves no longer need to go to the Fed Funds market to lend them out. They can get IEOR by default.
There are far fewer participants in the Fed Funds market than are in the repo market. The credit quality is the highest for these participants.
The agencies were large participants in this market - now they have shrunk their portfolio and with it their participation.
The largest participants now in the Fed Funds markets are the few entities like the Fed Farm bank that can't get IOER - that entices them to lend dollars(reserves) via Fed Funds - that's better than getting nothing. There's just not as much activity in this market - making it a less meaningful benchmark compared to repo trading and IEOR. The agencies (Freddie and Fannie) are also Fed Funds participants, but they no longer have the same level of funds to lend or borrow as they have smaller core portfolios.
The people who lend Fed Funds usually can't trade repo (I think)
There's some sort of credit enhancement to a Fed Funds trade (I think - I'm trying to get more info from our repo desk at work about this - I'll update this once I find out). I believe you have an enhanced position in the Fed Funds market compared to regular inter-bank lending.
There are also nuances to how different loans/repos settle. I'll try to get some more detail, but I think tri-party repo is EOD and bi-lateral repo is NOW. They all send cash over FedWire, but the timing makes a difference. I'm not sure when a general Fed Funds loan settles.
Also, in terms of counter-party access, often a person, like a money market fund, cannot lend to a user like a hedge fund, who needs to repo out their positions. This need to intermediate through adds friction to the process of arriving at one true rate.
These frictions make it hard for a Fed Fund lender, like the FHLB's to take advantage of the higher repo rate. They are lending into a smaller, more captive pool. Now if this went of for a few more days, then it probably would bleed in.
One other detail, who are the people with money to lend out for repo? That would be: banks with deposits, funds who have cash put them into money market funds that run repo programs (instead of just letting the cash sit at their bank or prime broker), and corporate treasury departments usually via money market funds.
One important thing to remember is that the total value of reserves and the total value of repo are really somewhat independent. There is about \$3.8t of Fed Balance sheet. I think there's about \$1.7t in cash out there somewhere (although I'm sure some of that is lost/destroyed). That gives you about \$2t of actual reserves. (Including foreign and Treasury accounts and the Fed). Now, you could have no one trading any repo, or people trading \$20t of repo. The amount of reserves doesn't directly correlate.
Another interesting thing. Users of the Fed' Foreign Reverse Repo facility usually have access to the general repo market. But, they were parking their money at the Fed in return for SOMA securities (the Fed's stash of Treasuries) instead of earning the higher rates. That's about \$300b of collateral - multiple times the \$75b that the Fed added this week.