I looked at the recipe for D-11, and the problem with it, is fairly diluted and with no silver halide solvent to clear the highlights, resulting in dense slides.

Look at the formula below it.. #203..

It's D-76 + Sodium Thiosulphate (a solvent), yet it is used straight including the solvent for only a slightly shorter time.

D-11 at 1:3 vs D-76 stock + Thiosulphate that is

1/6th the carbonate akali
1/6th the metol
Nearly half the hydroquinone
No solvent (vs 15g/L of sodium thiosulphate)

There's no way that D-11 recipe would give good slides without a solvent added, or highly elevated temperatures.

Get some sodium thiosulphate, or some sodium or potassium thiocyanate to add to it, then you'll see much better results.

Though I would use the D-67 recipe published myself (D-19 + thiocyanate) but with much reduced sulphite.