Stuck at some tolerance value

[meelih]

I modeled a workforce planning problem in GAMS.

The model generally assigns workers to shifts and to tasks according to their skill set and according to the given workload.

In the model we only have 5 regular workers and 17 subcontracted workers (they are used when needed and costs extra).

The problem is:

I am using 0.002 tolerance but the model stucks at 0.06 tolerance and works for hours without progressing in tolerance value.
The thing is, when i use 16 subcontracted workers(or less), the model gives the result in 2.3 seconds.

I know the stucking problem is not about the size of the model. I have different versions of the same model which are much more bigger in terms of worker numbers and they do not stuck.(They have the same constraints)

Do you have any ideas what may cause such a stucking problem in such a small model?
Any ideas, possibilities or suggestions are appriciated.
Thank you in advance.

[cladelpino]

Is this a MILP model ? Which solver are you using ?