There has been some discussion here in the past as to exactly what The Turk meant by this statement.

I am of the mind that he was referring to the fact that it was Don C's defenses and regard for his personal safety that was slipping.

After all, the families were in the midst of a long period of peace and prosperity at the time, and were not in their "always on guard" wartime posture.

Look at Sonny's wartime visits to Lucy. Bodyguards on every landing of the building, who completely surround Sonny as he leaves.

Contrast that with Vito's security the day of the assassination attempt: One bodygurad, Paulie Gatto, who is little more than a driver and substituted for by the ineffectual Fredo.

And then the Don wanders off, alone and unprotected, to buy fruit while Fredo waits in the car.

I believe that it is this lack of security and protection to which Sollozzo refers when he says "The Don was slipping. Could I have gotten to him five years ago?"

When taling about Paulie's absence that day, the novel provides a bit of dialogue between Sonny and Clemenza which further illuminates this point:

Sonny: How many times did he stay home the last couple of months?

Clemenza: Maybe three or four times. I always asked Freddie if he wanted another guy but he said no. There's been no cause, the last ten years things being smooth, you know.