This has to do with RLV implementation and is outside the control of the LULU Collar. Force-TP within a sim works a bit differently from between-sims.

When your owner TPs away to another spot within the sim, and you're leashed to him, an RLV call is made to force TP you. However, instead of a straightforward TP, your avie is actually being moved very quickly towards your owner. This means that, unlike the case between sims, obstacles between the two of you can prevent you from reaching your owner, causing the force-TP to fail.

Workaround

The workaround is simple. The LULU Collar detects that the force-TP has failed, and informs your owner to issue you a TP offer via his SL viewer. You will automatically be TP'd to him.