Ask if the traffic is ahead or crossing. If it's ahead, you can negotiate to remain at the current level by slowing down to 'his' speed/Mach no. If this is not an option, then I'd ask if I'd ask for a radar vector to avoid the traffic (possibly even a direct to YXZ) for separation - the controller will be able to see on her/his radar screen.
If still no joy, then you have no choice but to descend. Once down at the new level, ask when can you expect further climb - that way you can plan on whether you need to recompute your fuel or merely cruise for the next 20 mins (or whatever the case may be).
Finally, worst case scenario - lets say that you are stuck at your lower FL or altitude for the remainder of your trip knowing that you will not be allowed to climb, then your options are to request direct to ABC (short cut) and possibly make up some fuel at expense of arriving later due to your lower TAS/Mach no. or, get out the performance manual and calculate at that particular level/FL, what is your best endurance fuel flow.
Taking it further....now lets say you have exhausted all possibilities and ATC is still not accommodating you - now you can get an update of your destination's weather and assess whether at your revised ETA if the weather is still AT OR ABOVE landing minimums FOR THE APPROACH IN USE AT THE TIME OF ARRIVAL. If the weather is landing minimum or better, then you can legally continue to your destination using your alternate fuel as extra fuel.
The legality is to land with reserves, not alternate plus reserves.
You declare minimum fuel to ATC if at any stage of the flight, you anticipate that any other changes to the present flight route/conditions will cause you to land with less than reserve fuel. (note: this implies you now still have enough fuel to land with reserves intact. If you don't, then it is not minimum fuel you declare, but a fuel emergency)
So, if the weather at your destination is CAVOK and you filed IFR, since you are carrying an alternate, you are legal to use your alternate fuel after checking the weather (as previously mentioned).
If your destination weather is marginal....lets say it's RIGHT AT MINIMUMS you might find the weather whilst you are on the instrument approach reduces to below landing minimum as you pass the final approach fix...lets say you descend to 100 feet above the runway and cannot identify threshold markings, runway lights, edge lights - the landing environment [first 3000 feet (914 metres) of the runway], then you must perform the missed approach and assess whether you have enough fuel for another approach or merely fly to your alternate. This is an example whereby you would do what you can to preserve your alternate fuel as you may actually use it in the latter part of your flight.