Conformer generation for radicals
tldr: Manually or script dihedral angle changes, then use a GFN or another DFTB method to find any/all conformers

Without seeing the compound, it's really hard to know the problem. Most programs, including those you mentioned, have a set of rules to identify "rotatable bonds" and suggest likely dihedral angles.

As you might expect, such rules are derived for neutral, ground-state organic molecules.

There are a few options:


As suggested by Jan Jensen's comments above, you can use RDKit, Open Babel or another tool to drive your own dihedral angle search.
As suggested by Martin's comments above, there are some tools in Grimme's xtb  program to sample conformations. These will use the GFN1 or GFN2 approximate density functional (pretty much DFTB + dispersion correction) to evaluate energies.


I'd probably suggest a combination of both. From what I understand of Grimme's conformer search scripts, it calculates the vibrational modes of the molecule and samples geometries along vibrational modes. This is a good idea, although in some molecules it may not sample all minima (e.g., if you have many conformations in a large molecule, you might never "reach" the whole potential energy surface from any given initial geometry).

So you can change the dihedral angles using your program of choice, generate a few initial structures, then use xtb to optimize the geometries and confscript to sample a few more geometries. 

I feel pretty confident that in small to medium geometry molecules, this will do a good job.

A few notes:


If you're sampling a radical, you definitely want to use a good DFT or quantum method to evaluate the energies. I doubt any current force field will do what you want.
We haven't finished our evaluation, but right now, GFN2 is performing better than any current method on conformer energetics for neutral organic molecules. We haven't checked radicals, but I would trust it more than any other current "fast" method.
If you don't have the xtb package, the method and conformer sampling scripts are now available as part of ORCA 4.1 too. There's a description of this kind of conformer sampling in the manual when discussing prediction of NMR spectra.