Good answer. Montgomery's only real deficiency as a commander was an unwillingness to move until absolutely ready. This led to his success in Africa and much of his success in Europe, but it also led to his failure to end the war in 1944.
Patton's success in breaking out of Normandy was planned by Montgomery, who chose to use his own (British and Canadian) troops to tie down the majority of available German troops to Patton's north and west. However, Montgomery's most ambitious plan didn't turn out as well, Operation Market Garden. The plan was to drop paratroopers on a succession of four bridges across the Low Countries and drive a motorized column through to seize the bridges and relieve the light troops on the ground. The objective was to force a decision by Christmas.
Unfortunately, the plan depended on using all available air transport for the drop, and although Eisenhower initially agreed he was worried. Patton persuaded him to cut the transport, so the paratroops were landed over three days instead of one. But all four bridges had to be assaulted at once for the plan to succeed, so all four targets were attacked with insufficient force
The farthest bridge was at Arnhem, in the Netherlands. The first air assault was made 17 September, but Mongomery's lack of speed finally caught up with him. Although the paratroopers at Arnhem held out until the 25th the Allied forces could not reach them in time to secure the crossing. The pace of warfare in the Second World War had simply sped up so much that even those commanders thoroughly familiar with mechanized forces were sometimes too slow, although Patton and Eisenhower must take some of the blame for the insufficient air transport.
Motgomery was an excellent field commander, but his personal manner put many off, and his failure to cut the war short at Arnhem tarnished his reputation somewhat. He was still one of the most perceptive and successful commanders of the war.