TAMPA — Remember a month ago when Lightning coach Jon Cooper said the penalty kill had been a "sore subject" for a while?Well, it still is.But now it has advanced to a significant concern.The biggest issue Tampa Bay faces before the playoffs is improving its defensive play. And the porous penalty kill is a key part of that. The Lightning allowing two more power-play goals in Tuesday's 7-4 home loss to the Senators. The seven goals matched a season-high against Tampa Bay.The Lightning has given up at least one power-play goal in 10 of its past 12 games. It prevented a shorthanded goal only 68 percent of the time during that stretch and ranks 26th in the league this season (see chart)."It just keeps going into the net," said associate coach Rick Bowness, who runs the PK. "We'll fix it, let's put it that way."NHL penalty kill leaders Rank TeamPK%▼1San Jose Sharks84.902Los Angeles Kings84.123Colorado Avalanche83.684Winnipeg Jets82.985New Jersey Devils82.8826Tampa Bay Lightning76.8627Ottawa Senators75.8828Montreal Canadiens75.6629New York Islanders75.3830Philadelphia Flyers74.7531Edmonton Oilers74.64League Average80.09Provided by Hockey-Reference.com: View Original Table, Generated 3/14/2018.What hast gone wrong?Just last month, the Tampa Bay Times went to video to show how it has been a perfect storm of problems. Tampa Bay is one of the league's worst teams in faceoffs (47 percent) and second to last in shorthanded draws (41 percent).On Tuesday, the Lightning gave up one power-play goal on a perfect shot by Ottawa's Ryan Dzingel with seven seconds left in the man advantage.Dzingel picks the top corner for his 20th goal of the season. #GoSensGo pic.twitter.com/YlAtSW8eoP— Ottawa Senators (@Senators) March 14, 2018On the second, Lightning defenseman Anton Stralman had the puck on his stick late in the penalty kill, but he quickly lost it. Moments later, Marian Gaborik cashed in following a scramble. "Make one mistake," Bowness said, "and it's in the back of the net."Dzingel and Gaborik hook up for a nifty power play tally against the Lightning. #GoSensGo pic.twitter.com/hh6D0ghyiB— Ottawa Senators (@Senators) March 14, 2018"It's just getting the job done, it doesn't matter how it happens or why it happens," said wing Chris Kunitz, a four-time Cup champion. "For whatever reason lately we're not working hard enough to get the bounces to help our way. We have to be more committed to doing everything we can, you've got to outwork that power play."It's just mind-boggling sometimes how it works and how it doesn't. We just have to get better."But why is this a big problem?In the playoffs, you can get by with an average power play, not a weak penalty kill. Of the past 13 Cup champions, only one had a penalty kill below 80 percent in the playoffs (Blackhawks at 79 percent in 2014-15). Two have been above 90 percent. Seven have been at least 85 percent, including the 2003-04 Lightning Cup-winning team (85.5 percent).To see why, take the teams the Lightning likely will play in its road to the Stanley Cup:NHL power-play percentage SpecRankPP%▼1Pittsburgh Penguins25.894Boston Bruins22.025Washington Capitals21.896Toronto Maple Leafs21.81League Average19.91Provided by Hockey-Reference.com: View Original Table, Generated 3/14/2018.And that's just in the Eastern Conference. Out West, Nashville is seventh overall at 21.6, and Vegas — which scored four power-play goals in a victory over the Lightning in December — is eighth at 21.5.The Lightning has tried to fix the penalty kill. It has sprinkled in new personnel, including rookie Anthony Cirelli. Acquiring defenseman Ryan McDonagh from the Rangers on Feb. 26, not to mention forward J.T. Miller (strong on draws), was expected to help. Getting injured two-way forward Ondrej Palat back late this month could, too.Bowness said while structure hasn't improved, other issues creep up."When there's a shot and (structure) breaks down, there are reads you have to make," Bowness said. "Sometimes our reads have been the wrong reads. It's as simple as that."In Tampa Bay's Stanley Cup final season in 2014-15, it was tied for seventh in PK at 83.7 percent. The next year, when Tampa Bay advanced to the conference final, it was seventh again at 84 percent (86.6 in playoffs).While the Lightning dropped to 13th last season, the fact it was missing Ryan Callahan was thought to be part of it. But there has been a free fall even with a healthy Callahan and the July addition of shot-blocking specialist Dan Girardi.Each time the Lightning has gone on a lengthy playoff run, it has ranked in the top-10 in penalty kills. It's Stanley Cup champions in 2003-04 ranked 10th in the regular season. In 2010-11, when the Lightning reached Game 7 of the East final, it was eighth (92.3 percent in playoffs).And if the Lightning wants to play deep into this summer, it'll have to dig deeper on penalty kills.