Welcome to the frustrations of using JFETs.
A JFET cannot pass more current than Idss. It does that when the gate is at the source voltage, or Vgs = 0V.
A JFET turns completely off when the Vgs is more than the cutoff value on the data sheet, Vgsoff.
In between the two extremes of Id = Idss at Vgs=0 and Id=0 at Vgs=Vgsoff, the current varies by the transconductance, gm. The units of gm are 1/resistance or amperes per volt. The only thing keeping you from just specifying an Id by using Vgs times gm is that Idss, Vgsoff, and gm all vary by between 3:1 and 10:1 within the same type number.
So here's what you gotta do if you are replacing a JFET.
1. Calculate the current you need to flow in Id. If you have a spec of drain voltage, then convert that to an Id by using the drain resistance to figure it out.
2. Pick a JFET that has Idss bigger than the Id you need. If the JFET's Idss is smaller than the Id you need, you'll never get there. So test for Idss> Id.
3. From the JFETs with Idss big enough, set up a test circuit to measure the Vgs that gives you the correct Id. Swap in the JFETs that have big enough Idss and diddle the Vgs until you get the correct Id. Measure the Vgs. If your circuit can supply that Vgs, you're there. If not, keep looking.